You insert the desired letters into a stamp and use the stamp with a stamp pad to provide your return address or any 1 - 3 line set of words and numbers.  Note that the word and letters are mirrored when stamped on a paper / envelope, so insert the letters from right to left.

The is a 4 mm high emboss in each slot in the stamp that provides a light press-fit for the letters and numbers so they stay put, but can be moved/removed as needed.

I used PLA, but the more flexible PTEG may work better.