Commercial tills also integrate with a huge bunch of other specialized periphery. Payment terminals are an important category (and an ugly one, because each country has its own, and there are many different protocols in use), but receipt printers, barcode scanners, RFID systems (to perform cashier authentication), cash drawers, checkout scales, cash management systems, RVMs (reverse vending machines; those things taking bottles back at the store) and whatever other, often custom hardware big retail chains come up with (think store signaling systems for example).
They also need to take fiscal requirements into account: in a growing number of countries, cash register systems have to satisfy fiscal requirements and must perform some sort of transaction signing and reporting to fiscal authorities, or integrate special hardware which gets all transactions and prints out the receipts instead of a "normal" printer on which you can print anything. This is an ugly topic that easily keeps several full-time employees busy, because fiscal requirements are mandated by law and thus are moving targets. The US doesn't have this at the moment, but compensates by having a rather special and complex tax system ;-)
And then there's the calculation of prices modified by promotions (think coupon codes, time-controlled rebates, such stuff) which is an entire topic on its own, especially when taking the interplay of promotions with sales tax calculation and rounding logic into account (not all countries round to .01 cents, some don't have anything smaller than 0.05, and some retailers voluntarily decide to round to multiples of 0.05 in order to eliminate handling effort with small coins).