First, thanks for being such a great customer! We love hearing that shops are happy using our app and want to continue using it in more stores, and we really appreciate your support.
Unfortunately we don't have a 'multiple' or 'combined' billing method in place to either bill separate Product Customizer installations together, or offer quantity discounts for installing the app in multiple shops at this time.
While offering a multi shop plan option is something we would definitely consider if it were easy with the Shopify billing API, it's just not particularly straightforward at the moment and would require some intensive changes to the app's billing. We'd also have to find a way to tie shops together based on the owner's information, while right now all of the data in the app is tied to the myshopify URL, as we know this will represent a unique shop.
With so many upgrades to existing functionality and new features in progress, we're not comfortable taking time away from improving app functionality to spend time instead on adding complex plan options. Basically, we feel that making our apps better provides more value to our customers than spending time on the way the billing happens.
Our hope is that the app adds enough value to your shops that it makes sense to have them installed in each of them. While a multiple-shop billing option is something we'll absolutely be revisiting in the future, we'd like to make Product Customizer as great as possible first.