We have used three different types of tape to seal our sandwich bags and attach the string to the card package: Scotch tape, masking tape, and packing tape.
We have generally used Scotch tape because it was most available.
Masking tape was used when we were out of Scotch tape.
We found that transparent packing tape works the best because it is about as wide as the card and envelope, therefore, we only have to use one strip of tape to seal the envelope and attach the string to the balloon. Clear tape makes it easy to see and read the contents in the envelope package.
If you have any questions or comments about tape, please e-mail me.