Probably not too relevant in this context of being cheated with a small amount for a large number of times, but if you ever feel that a company may swindle you

Probably not too relevant in this context of being cheated with a small amount for a large number of times, but if you ever feel that a company may swindle you for a larger amount unexpectedly -- you should use a disposable credit card (gift card?) that can be bought at the local store.