Online tax invoice software — issue bills that are compliant with the Thai Revenue Department, free to start

BillsOS is online tax invoice software for Thai freelancers and small shops. Issue full and abbreviated tax invoices, receipts, quotations, and 50 Tawi withholding-tax certificates in the format set by the Revenue Department, with a PromptPay QR that fills in the exact amount for you. Money lands directly in your account. Start free, no credit card.

By the BillsOS team · Updated 6 June 2026

This article provides general information for understanding only and is not specific tax or accounting advice. If unsure, consult a licensed accountant or the Revenue Department.

---

What a legally compliant tax invoice must contain

Before choosing billing software, the most important thing is that the document it produces is complete under the Revenue Code. If fields are missing, your buyer may be unable to claim it as input VAT, and you yourself may have problems during a Revenue audit.

A full-form tax invoice, under Section 86/4 of the Revenue Code, must contain at least the following:

1. The words "Tax Invoice" (ใบกำกับภาษี) shown prominently
2. Name, address, and tax identification number of the registered operator issuing the invoice
3. Name and address of the buyer of goods or recipient of services
4. Sequential number of the invoice, and the book number (if any)
5. Name, kind, type, quantity, and value of the goods or services
6. The amount of VAT calculated on the value of goods or services, clearly separated from the value of the goods
7. The day, month, and year the invoice is issued
8. Other particulars prescribed by the Director-General of the Revenue Department (e.g. "documents issued as a set" in certain cases)

Note: a full-form tax invoice must clearly state whether the displayed price includes or excludes VAT, and must show the VAT amount separately.

BillsOS lays out all eight of these fields in a single form and calculates 7% VAT on a separate line automatically. You just enter the name, address, and tax ID correctly — the system arranges the rest.

Full-form vs abbreviated tax invoice

In short: if your customer is a company, or anyone who needs to claim input VAT, always issue the full form. The abbreviated form suits walk-in retail shops selling to many ordinary consumers — and the law grants this right only to retail businesses. BillsOS supports both and switches between them in a few clicks.

Tax invoice ≠ receipt ≠ invoice

Many people confuse these three documents:

• Tax invoice — a tax document under the Revenue Code that shows the VAT amount; only VAT-registered operators may issue it.
• Receipt — proof that payment has been received; can be issued even without VAT registration (non-VAT businesses often issue a "cash bill").
• Invoice / billing note — a document requesting payment before it is received; not itself a tax document.

In practice many shops issue a combined "Tax Invoice / Receipt" on a single document when receiving cash — a format BillsOS supports.

---

You must register for VAT before you can issue a tax invoice

This is important and often missed: only operators registered for VAT may issue tax invoices. Under Section 86 of the Revenue Code, a registered operator is required to issue a tax invoice every time they sell goods or provide services.

• Operators with revenue exceeding 1.8 million baht per year are required to register for VAT (filing form Phor.Phor.01 within 30 days of the date revenue exceeds the threshold).
• Those not yet registered for VAT (below the threshold, or simply not registered) cannot issue tax invoices — issuing one without the right to do so carries legal penalties.
• If not VAT-registered, issue a receipt / cash bill instead, which the buyer can still use as evidence of an expense (subject to Revenue Department conditions).

BillsOS has a VAT on/off switch: if you are not yet VAT-registered, leave it off and issue receipts or quotations. Once you register, turn it on to issue full or abbreviated tax invoices — no need to change software.

BillsOS is a document-creation tool — it does not file taxes for you and is not a tax adviser. If you are unsure whether you must register for VAT, consult an accountant or the Revenue Department.

Read more: VAT guide for freelancers and small shops →

---

What BillsOS does for you

• Issue full and abbreviated tax invoices in the Revenue Department format, with all fields under s.86/4 / s.86/6.
• Receipts, quotations, and 50 Tawi (withholding-tax certificates) in one app.
• VAT on/off — works for both VAT-registered and non-registered businesses.
• PromptPay QR with the exact amount filled in — customers scan and pay the precise total, and money lands directly in your bank account. BillsOS never holds your funds and charges no per-bill fee.
• Bilingual Thai/English, suited to international clients.
• Share via LINE — send a bill to your customer instantly.
• CSV export (Pro plan) so your accountant can post the entries easily.

See also: How exact-amount PromptPay QR works → · Issue 50 Tawi →

Who it is for

• Freelancers whose corporate clients ask for tax invoices and withholding tax — 50 Tawi is available from the Freelancer plan.
• Online shops / small shops that want exact-amount PromptPay and to send bills via LINE.
• Businesses that have just registered for VAT and need to start issuing compliant tax invoices.
• People not yet VAT-registered who simply want professional receipts and quotations.

---

BillsOS compared with FlowAccount and PEAK

Prices as of June 2026 — check the latest on each provider’s own page. All figures are the "entry plan that includes tax-invoice features".

Where BillsOS wins: it is built with PromptPay at its heart (amount filled in, money straight to your account, no per-bill fee), it is the simplest option for anyone who just wants to issue compliant bills and get paid, 50 Tawi is included in the entry plan, and pricing is transparent with a free tier you can try without a card.

Where competitors fit better (frankly): if you need a full accounting system — general ledger, financial statements, multi-channel e-commerce integration, or a team of several accountants — FlowAccount and PEAK have more complete modules. BillsOS focuses on billing plus getting paid, as fast as possible.

---

How to get started (3 steps)

1. Sign up free at /signup — no credit card.
2. Enter your business details (name, address, tax ID) and turn VAT on/off to match your status.
3. Issue a tax invoice + PromptPay QR, send it to your customer via LINE, and receive money straight to your account.

---

แหล่งอ้างอิง / Sources

Primary sources from the Thai Revenue Department (rd.go.th) and secondary sources (provider pages and accounting guides). Competitor prices surveyed as of June 2026 and may change.

• Primary — Revenue Department: Sections 85/86 Revenue Code (s.86, 86/4, 86/6)
• Primary — Revenue Department: Tax invoice handbook (PDF)
• Primary — Revenue Department: VAT Notice No. 39 (s.86/4(8))
• Primary — Revenue Department: SME tax handbook (1.8M VAT threshold, PDF)
• Secondary — provider page: FlowAccount — pricing page (surveyed June 2026)
• Secondary — provider page: PEAK Account — package pricing (surveyed June 2026)
• Secondary — accounting guide: iTAX — receipt vs tax invoice
• Secondary — accounting guide: BeeAccountant — abbreviated tax invoice
• Secondary — accounting guide: PEAK — revenue over 1.8M but not VAT-registered

© 2026 BillsOS · a product of Cavastir