Helpers here are not required to join the union unless they are hired on after Peak.
It's hard to know anything with certainty when it comes to UPS and the Teamsters, but I assume the default position is that delivering packages is Bargaining Unit work and therefore is to be done exclusively by Bargaining Unit members. So, rightly or wrongly, in a non-RTW state, a Driver's Helper should be paying dues and initiation fees (or Agency Fees in the case of a Consciencious Objector.)
However, a Local can always, within limits, charge a lesser dues rate or smaller initiation fee; and besides, if the Helper is only working a few days, the Union may simply not catch up to him before his short Peak Season is over.
I wouldn't think anyone could get a refund if they actually worked a day during the month in question.
Anyone else know for sure how helpers are treated in your area?