Thanks for reaching out to Jotform Support. Yes, unfortunately, the feature you're looking for isn't available at Store Builder right now. We've gone ahead and escalated your request to our developers, but when or if it's developed depends on their workload, how viable it is, and how many other users also request it. If there are any updates, we’ll circle back to this thread and let you know.
As a workaround for that, you can create an Order form in Form Builder, set up the special pricing option, and then add the form to your App Builder. Let me show you how:
In App Builder, click on the Add Element menu on the left side of the page.