Can I use google fonts on prints where buyers could ask for some changes e.g add their names on.
No problems as long as the font files stay untouched.
One theoretical possibility to get into troubles exists: If the place where customer's name or another change is inserted is inside a Google font file which is embedded to the print PDF, legal problems can arise. Copyright owners have allowed Google to distribute fonts freely for creating printed and displayed texts, but making new versions is limited and there's rules how to credit original creators and use font's name. Read the license! Get a lawyer if you cannot understand it.
Another thing: Google font files cannot be sold as is, but you can embed them into a PDF which can be for sale.