We have analyzed your requirement and we are ready to help you. Can you please share your comments for following our points.
1. Customers want to upload the disability certificate from your store while purchasing the items?
If customers want to upload the certificate then you need to do additional customization from your store.
Please feel free to contact Us at firstname.lastname@example.org
i'm also here to your help . please write at email@example.com or PM me.
Unfortunately there is no way to do it on the clean installation. Additional code modifications are required. Feel free to contact us
On clean install the only way (long way) is playing with 3 globals options to match the correct price with negative price or negative % ratio , is not perfect but in this way I have the correct price .
Another way is create 3 cupon code in -% and apply to 3 differents product list but not easy , I prefer checkbox button in product detail page.
Upload the disability certificate increase abandoned checkout , I put file upload option but not requiered ... the client send me later by mail the certificate or with wasap , fax ect...
I must finish in first time the product catalog all your website are in my favorite .