Little bit of background: a few weeks ago, my friend/coworker's sister, we'll call her Lila, opened up a shop with her boyfriend, let's call him Mark, about things that generally interest me. I've helped them as much as I could to set up the shop before it opened, because I thought it was a nice place, and I generally like their family. We weren't friends before, but since then I started visiting them on their shop regularly (about twice a week), just to chit-chat, look around what's new, or just hang out.
However, since they seem to be very grateful for my help (even though I don't think it was really that much), and because they're generally kind and nice, they'll often offer me things from the shop for free, or with a great discount, when I show interest in that thing or intend to actually buy them. The values of these things are in the tens of dollars. As much as I appreciate their generosity and friendliness, I'd rather pay full-price for the things I get there, since it is still a business and I feel like I shouldn't have privileges over other clients, not to mention I can't help but think of how these discounts might sum up at the end of the month on their profits.
I'd like to tell them that I don't want free things/discounts just because I'm friends with them, but I don't know how to communicate it in such a way that doesn't make me sound ungrateful, uncaring or rude. Sometimes Lila would offer me something I wanted to buy for free, to which I awkwardly insisted in paying full price. Sometimes Mark would offer me a product with a discount, which I would often just end up accepting, to avoid any embarrassment. Note that I'm very introverted, so I have a hard time communicating those things without it coming off weird.
How can I decline a friend's continuous acts of generosity without being rude or ungrateful?
EDIT: Some clarifications that I should have considered before posting: this is a small business and they're just starting, so I don't think they're as well-established as to be able to provide free/discounted items all the time. Maybe in the future it might not be such a problem, but for a new and small shop, I'm not so comfortable with it.
I've been friends with Lila's sister for years, but not with Lila herself (or Mark, whom I had just met) until I started helping them with their shop, so I don't have as much intimacy with them as I'd like to in order to approach this subject without causing awkwardness.
The reasoning behind their gifts are both because I helped them and because they want to be kind with a friend, so it's not as much "bothering" to me as it is guilt-tripping. The discomfort comes from me trying to be fair to them business-wise without it sounding like I don't appreciate their kindness friendship-wise.
Thanks everyone for the answers! I talked to them and insisted on paying full price, and they insisted on giving me discounts, especially since I was always there to help them, and because we're friends. So we've settled for reasonable discounts only.