Update: So my BF texted his dad Saturday morning to say we were planning to come buy our tree that day, and his dad texted back that he'd be at the lot all afternoon, and then (I think) my BF texted something to the effect that we wanted to pay for our tree, and there was no response to that. When we got to the lot, it turned out his dad and all three siblings were there, and his youngest brother and sister (both tweens) were very eager to help us find a tree. As it happened, after we'd almost settled on one, the brother pointed out another tree that had JUST been put on display, and didn't have a price tag yet. It was a very nice tree, so we ended up taking that one.
His dad put it on my car, and as he was finishing up I asked how much I owed, and we said again that we weren't expecting it for free and would be happy to pay full price, and he said how about half off? We said that sounded good and I paid, and then I gave tips (very nice ones, in part because of what bills I happened to have in my wallet) to his brother and sister, which my BF said made their day.
As is my natural tendency, I worried a bit afterward that I'd screwed up somehow, but my BF said I was fine, and it was definitely a lot more fun saying hello and buying a tree with his family than from some random guy at another lot, and we have a great tree! So it all seems to have worked out. Thanks again for the advice, it eased our anxiety a lot.