You can use an expression to automatically pad a string or number to a certain number of positions, by using a combination of the RIGHT() function and by concatenating the string with the characters you want to pad with.
Example: Padding a string with dots (periods) to 20 positions:
RIGHT("...................." & [DataField], 20)
Result (if [DataField] contains "Example"):
.............Example
Example: Padding a 5-digit zip code with zeroes, to 5 positions (this is useful if PrintShop Mail sees the zip code as a number and removes the zeroes automatically):
RIGHT("00000" & [ZipCode], 5)
Result (if [ZipCode] is 1234):
01234