I would suggest a simple reply sticker printed, as I have found this an annoyance to arrange, and also that I've had to pay at times to return, when I have bought two items to make sure I get the best fit.
Also a voucher or a "Share and Win" given daily or weekly to a boy and girl via a selected hastag on insta or FB have that credited to your account, or receive an emailed special code, for 30 day use.
goes without saying a discount for your birthday also that lasts 30 days.
I suggest 30 days, some people (me) are paid monthly.