A point-of-sale system is the only piece of software that touches every dollar the business takes. Chosen well it tells you what sells, when, and to whom, and hands clean numbers to your accounts without anybody typing them twice. Chosen badly it is an expensive till that holds your sales history hostage.

The cost that matters is not the one on the website. It is the processing rate on every transaction for the next five years, and a difference of half a percent on $400,000 of card takings is $2,000 a year, every year, for a decision made in an afternoon.

Work out what you take by card in a year, and roughly what an average sale is. Everything below is judged against those two numbers, and without them you will compare monthly fees and ignore the thing that actually costs you money. The per-transaction fee is charged on the number of sales rather than the amount, so where the average sale is small a few cents a sale can cost more than the percentage does.

Write down the three things you need it to do, and be strict. Most businesses need to take payment reliably, know what sold, and hand figures to their accounts. Features beyond that are worth having and are not worth choosing on.