1. Look at the docs for the wallet or run a google search.
2. Tills normally run windows operating systems, why not just install bitcoin core on those (encrypting it giving the manager/store owner the access to the password and everyone elese able to only generate addresses). You could also use electrum wallet but they won't be as reliable as bitcoin core as you're relying on a third party (who is still fairly trustworthy though)...
3. If this were the case, some sort of payment processor like coinbase might be better to process consumers transactions. Alternatively, the store owner could keep their profits in bitcoin (or half of their profits) and turn the rest to fiat in order to use it to buy more stock/pay staff etc...