Of course most of them are using Square or the equivalent which are modern online payment systems, despite the rustic small-biz nature of Square’s customer = your produce seller.
A side question for the OP is how many and how much value are you talking about, and how much trouble is it worth to you to extract that value if it isn’t trivially easy.
[semi-related story]
My late first wife was a lawyer. And eventually became fully home-based after years of renting an office. We had quite the supply of pro office equipment. back in the day she did lots of snail mailing of large envelops full of stacks of heavy paper, so we had lots of postage stamps. First class postage has always been one price for the first ounce, and a different price for subsequent ounces. The two numbers changed over the years, and not always in sync, but that idea was and is constant. And 9x12s cost a lot more to mail than ordinary envelopes and we mailed a lot of thick multi-ounce 9x12s.
Anyhow, after 30 years of this we had quite a collection of oddball denomination stamps. We didn’t buy in huge quantities to begin with, but you’d end up with 12 of the 22cents, 17 of the 24 cents, and 5 of the two cents you’d bought to upgrade your leftover 22s to 24s when the price bump first happened. etc.
Over time of course e-everything took over and snail-mailing multi-ounce items became more rare. Then her business dwindled along with her health. And all along we’d been storing all the postage stamps in a box and I’d try to use up a couple or few of the lower denomination ones whenever I’d snail mail something.
When I was cleaning out the house to move after she died I roughly inventoried the stamps. Came to about ten dollars worth of small stamps and IIRC about ten $1.00-plus oversized rate stamps. I threw away all the ones that were not current denomination. (And yes, we’d long ago switched to first-class forever; it was just the leftovers from before then, and the second+ ounce stamps left behind. And the dollar+'s for the big envelopes)
It felt weird to throw away what’s effectively US currency, but the effort / reward ratio of storing then extracting that small value had slowly gone negative over the years unnoticed by me. Not my problem any more. Feels good now.
The OP doubtless has more money in her gift cards than I had in stamps. But if it’s too hard to extract, it’s too hard to extract. I have caught myself doing silly things to redeem a $1 coupon. Because way back when I was a kid, a dollar was real money. Not so much anymore. Something to think about.