Articles | Updated 29 October 2026
What Is a Purchasing System? What a Purchase Order Is For, Step by Step
A purchasing system looks after buying for the business, from sending a purchase order to the supplier and receiving goods to recording what you owe and paying. A purchase order, or PO, is the document a shop sends a supplier to confirm what it will buy, how much, at what price and when it should arrive.
What purchasing documents are there?
One purchase usually involves 4 documents. Some small shops skip the first one.
Purchase requisition (PR)
Issued by: Staff in the shop
Used for: Asks a manager to approve buying
Purchase orders (PO)
Issued by: The shop sends it to the supplier
Used for: Confirms items, quantities, prices and delivery date
Goods received note (GRN)
Issued by: The shop records it when goods arrive
Used for: Confirms how much actually arrived, and adds it to stock
Invoice and tax invoice
Issued by: Issued by the supplier
Used for: Bills you, and lets you claim input tax
| Document | Issued by | Used for |
|---|---|---|
| Purchase requisition (PR) | Staff in the shop | Asks a manager to approve buying |
| Purchase orders (PO) | The shop sends it to the supplier | Confirms items, quantities, prices and delivery date |
| Goods received note (GRN) | The shop records it when goods arrive | Confirms how much actually arrived, and adds it to stock |
| Invoice and tax invoice | Issued by the supplier | Bills you, and lets you claim input tax |
Why have a purchase order?
- Prices are agreed in advance, so when goods arrive you can check the bill matches
- You know what has already been ordered, so you do not order twice
- You can check whether everything has arrived and how much is still outstanding
- The owner can approve large purchases before they happen
- The product cost you use for pricing comes from the price actually agreed
Example: an order that does not arrive in one delivery
Say a chilli paste factory orders 50 kg of dried chillies at 300 baht a kilogram: 15,000 baht before tax, VAT 1,050 baht, 16,050 baht in total. The supplier delivers in 2 rounds.
Issue the purchase order
What happens: Order 50 kg at 300 baht
Result in the system: Purchase order waiting for goods
First delivery
What happens: 40 kg arrives
Result in the system: Stock up 40 kg, payable of 12,840 baht, purchase order partly received
Second delivery
What happens: Another 10 kg arrives
Result in the system: Stock up another 10 kg, a further payable of 3,210 baht, purchase order fully received
Payment
What happens: Transfer 16,050 baht to the supplier
Result in the system: Supplier balance cleared, input tax of 1,050 baht goes into the input tax report
| Stage | What happens | Result in the system |
|---|---|---|
| Issue the purchase order | Order 50 kg at 300 baht | Purchase order waiting for goods |
| First delivery | 40 kg arrives | Stock up 40 kg, payable of 12,840 baht, purchase order partly received |
| Second delivery | Another 10 kg arrives | Stock up another 10 kg, a further payable of 3,210 baht, purchase order fully received |
| Payment | Transfer 16,050 baht to the supplier | Supplier balance cleared, input tax of 1,050 baht goes into the input tax report |
How is a purchasing system in an ERP better than keeping records yourself?
On paper or in Excel you have to note what you ordered, how much arrived and who you owe, then update stock and the books again. With purchasing inside an ERP, one receipt of goods adds stock, works out cost, records the supplier payable and puts input tax into the report. When payment day comes, you can see which bills are due.
Shops where staff order on the owner's behalf should have approval rules, for example purchase orders over 20,000 baht need the owner's approval first. The system does not let the person who placed the order approve it themselves.
The purchasing system in GORINY
GORINY issues purchase orders in baht or foreign currency, marks them as sent to the supplier, receives goods in parts, and uses the purchase order price to cost stock and record the supplier payable automatically. For suppliers who are not VAT registered, the amount due has no VAT added. You can set multi-step approval rules for purchase orders and supplier payments, the person who submits cannot approve their own request, you can see each supplier's latest purchase price, and you can withhold tax when paying. GORINY does not yet have purchase requisitions; shops that want a requisition step can use purchase order approval instead.
Frequently asked questions
- What is a purchase order?
- A purchase order, or PO, is a document the buyer sends the supplier, stating the agreed items, quantities, prices and delivery date. It serves as evidence of the order.
- How is a purchase order different from a purchase requisition?
- A purchase requisition is used inside the shop, for staff to ask a manager to approve a purchase. A purchase order goes out to the supplier to actually order the goods.
- Can a purchase order replace a tax invoice?
- No. A purchase order is issued by the buyer, while a tax invoice must be issued by a VAT-registered seller. The shop uses the seller's tax invoice to claim input tax.
- Does a small shop need a purchasing system?
- If you buy from several regular suppliers, buy on credit, or have staff order for you, a purchasing system keeps you from ordering twice, paying the wrong amount or forgetting to pay.
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