What should a sales invoice contain?

A sales invoice does not need to be long. It needs to do two things: let the buyer know what they paid for, and let the seller prove the transaction happened. Leave out any of the items below and you will have to ask someone later - and sometimes there is nobody left to ask.

Seller details: shop or business name, address, phone number. Add a tax code if you have one; customers who need the invoice for their books usually require it.

Buyer details: the customer name. For a walk-in customer, a name is enough. For a business customer, include the company name and tax code, since that is what they use to record the expense.

Invoice number and issue date. Numbers should increase and never repeat. The issue date is the date revenue is recognised, not the date you got around to writing it down - a date that lands in the wrong period can throw off two months of books at once.

The line items: description, quantity, unit price, line total. Describe goods specifically enough that another person can understand them. Labelling everything as "goods" means nobody can tell later what was actually sold.

The total, plus any discount, tax or delivery fee stated clearly. Most important is the FINAL amount the customer owes - do not make the reader add it up.

Finally: payment method and whether it has been paid. An invoice that does not say whether money changed hands cannot be used to chase a debt or to reconcile the till.

One detail that is easy to overlook: keep a copy. Once a paper invoice is handed over, there is nothing left to check against. Export an extra PDF and file it by month - the cheapest possible insurance against a future search.