I went to a local Turkish supermarket recently. The majority of the staff are Turkish and clearly speak English as a second language although their English is leagues better than my Türkçe!!!

After I passed through the checkout and paid for my goods, I nearly said teşekkürler dostum but didn't; partly because I was not very confident about my pronounciation but mostly because I was worried than 'dostum' would seem too familiar in this situation.

What should I have done in your opinion?
And why?

While passing the money you could say: buyurun

If the shopkeeper is giving you either your cashback or your items you could say: teşekkür ederim.

When walking in or before leaving you could say: iyi günler