custom inventory quantities for fabric yardage

I am struggling with my current ecommerce website and am looking for a new platform and developer.

The problem that I’m having is that no one knows how to code a special app or extension to sell fabric and there is nothing available out-of-the-box that I can find. I would think this would be straightforward but no. It has stopped the hiring of several web developers and usage of many solutions.

I’m appealing to the WP community to see if something like this exists or if it’s straightforward to code. It’s ok if it takes time to code I just need to see if it’s possible on WP.

I sell yarn and fabric. Yarn is sold by the unit, i.e. integers. No problem.

Fabric is priced by the yard and sold by the half yard with a half yard minimum. In other words I have fabric that is listed at $25 per yard and someone wants to buy 2.5 yards or $62.50. I want the customer interface to be clean and easy to navigate and when 2.5 yards is purchased I want my inventory to reflect correctly. i.e. restricted floating point numbers.
When the next customer goes to buy they may see in the quantity 15.5 yards available.

I’m not looking for an alternate solution or a way to do this differently. I’ve tried different pricing structures (by the half yard) and they don’t work and only confuse my customer. A clean example of a fabric yardage interface is britexfabrics.com. Just put some fabric in your cart and you’ll see what I mean.

Appreciate any help or links to ecommerce add-ons that can do this.

p.s. solutions I’ve tried include: opencart, magento, volusion, ashop, shopify, bigcommerce, pinnacle, lemonstand. These do not have out-of-the-box and I cannot find apps or extensions or developer that can code this. Ashop, pinnacle claim that they can but I’ve tried these and they don’t.