We do offer this with our nostr:npub155m2k8ml8sqn8w4dhh689vdv0t2twa8dgvkpnzfggxf4wfughjsq2cdcvg integration, which enables a merchant to create a list of items with corresponding prices. I showed it to ~10 vendors and not a single one was interested. They don't want to put in the effort to set it up. And then if they ever change prices/products, even more work. We recently started getting some more traction using our POS Keypad which allows the merchant to generate bolt11s with the amount built-in. Still an uphill battle though. Pura vida.
Now in terms of experience improvements - while this is time efficient when I make the payment in parallel to them making the coffee, I find paying to lightning address QR annoying. The main annoyance is that I need to put down the total amount in my app.
Imo a better experience would be if the QR opens a page that either allows me to select from the list of products (e.g. Latte: 2000CRC, Espresso:1000CRC...), I just select those, tap pay and that opens my lightning wallet of choice. Even better would be if the website somehow magically knows exactly what I ordered (this would require coordination with some seller POS, so this more applies to restaurants, not ferias).
The website could show something like 
nostr:nprofile1qqs2n79n72kpnnqx65v5m5dvjv2dgaq6p9mhg3ycv4feymv3v5vpv3cpz4mhxue69uhhyetvv9ujuerpd46hxtnfduhszyrhwden5te0dehjuum5wghxxu30qyvhwumn8ghj7urjv4kkjatd9ec8y6tdv9kzumn9wshsfvyehu nostr:nprofile1qqs254dy0xkkjdxsl4u08k7cs52u689q67s3pqfwwyfcp4va7avcjdgpr9mhxue69uhhyetvv9ujuumwdae8gtnnda3kjctv9uq3vamnwvaz7tmjv4kxz7fwwpexjmtpdshxuet5qy8hwumn8ghj7mn09eehgu3wvdeqld6ps4 - just some ideas 💡 😉
Discussion
Yeah, that makes sense and I see it from the vendors perspective that they don't care about this and it just adds annoyance. The POS in the app solves the problem but it adds a bit more work for the seller during every purchase (and it happens synchronously, so it slows down the overal experience).
Is there some other way to do it if we think outside of the box? A person in the morning goes around the feria and collects the items they sell and inputs them in the POS QR somehow? The seller takes a picture of their "menu" (e.g. the coffee place in this example has printed and laminated menu they use every time) and the app could OCR it and provide the options? 😉 That's probably too much work for a single market, but on a bigger scale it may be interesting...