ask her. the free shirt idea is only a good one if he’s interested in giving you a free shirt—i can easily imagine a situation in which he would be looking at your potential purchase as a sign of support from you, rather than as an opportunity for him to give you a present, which may be interpreted as sucking up. it might be awkward for him to give you a free shirt if he doesn’t think you like the website (what is the website??).
but yeah, ask her. “do you think you & lenny are serious enough that i should invest in a soapee tshirt, marta?” should do the trick.