the QR codes are because i'll have 5 or 6 volunteers giving out a lot of products in a very short time on the day. imagine it more like tickets for an event which people will be lining up for and so they won't be able to manually check codes against a list fast enough.
Scanning something pre-generated with their phone is a much better solution than say groupon like vouchers. Is it possible?
Nick here from Shopify.
I took a look through the Shopify app store for you and did find a couple of apps which might fit with what you're looking for. The first one I came across is Shopcodes. Shopcodes enables your customers to quickly scan Shopcodes with any smartphone camera and purchase your products. It doesn't seem like there is a specific pre-sale aspect to it. QR code by Freeh is another one which seems to better fit with a coupon than a direct link to a product page or specific page on your store.
I'm not sure which apps you came across before, but I'm hoping either one of these two can do what you're looking for. The other potential option is to have something made custom for you. Reaching out to a Shopify Expert about having a custom QR code feature built is the best alternative to one of the apps. It would probably be more expensive but it would be tailored to you and your store's needs.
Hope this helps and gives you some options on the best next step to take with this.
All the best, Nick