The ink will scratch off of the shrinky plastic, so if you want to seal the charms, I've had the best success with dipping in polyurethane and letting dry overnight. However, I have also had some hairline cracks appear by the hole when i've tried this with tear drop shaped charms. So your mileage may vary.
You can also try painting with clear fingernail polish, several of my readers on Dabbled have said that worked for them!
Unless you want the colors to lighten considerably, don't seal with any sort of spray acrylic that contains acetone. Although, it may give you an interesting effect! (I have a whole section on that in my experiments