Polish tipping etiquette can be a bit confusing for foreigners. While in other countries it’s perfectly normal and even courteous to say 'thanks' when the wait staff comes to collect the bill, you’ll be startled to learn that in Poland uttering the word ‘dziękuje’ (thank you), or even 'thank you' in English, is an indication that you don’t want any change back. This cultural slip-up can get very embarrassing, not to mention aggravating, if you're later forced to track down your server and shamefully ask for your change. In order to avoid these situations, we strongly suggest that you only say ‘thank you’ when the service of your waiter/waitress is no longer needed and you are happy for them to keep all of the change. When that’s not the case, the word you need learn is ‘proszę’ (pronounced pro-sheh). Meaning ‘please’ in Polish, this is a vital social cue that translates to ‘yes, I want all of my change, no matter how small it is.’
Despite the fact that the standards of service in Poland have skyrocketed in the last decade, the average waiter/waitress in this country still only makes a paltry 8-12zł/hr, and a customary tip is still only 10% of the meal's total (though being a foreigner may make staff expectant of a bit more generosity). Those numbers look pretty low to us, so we strongly encourage you to reward good service when you feel it's deserved.
Finally, it is not common practice to add the tip to your credit card payment because the wait staff are then forced to pay tax on the gratuity; most likely you will not even have an opportunity to leave a tip on your card. For that reason, try to have some change handy so you can still leave a cash tip, or ask your server for change if you need to.