Online Store with File Upload Capability

I am a current Web Assist client using Super Suite and a few other solutions on various projects. I am really intrigued by the PowerStore solution for launching a store for our business web site, but here is the solution I need which Power Store can not provide currently. So if anyone can help me out with an elegant solution that is relatively easy to implement (I am a designer, not a programmer) I would be very happy.

We are a graphic design and digital printing business. I want to launch a store for selling display solutions - banner stands, retractable POP, popup booths, etc. Creating the store is relatively straightforward (at least I hope so with the Power Store solution) for the products BUT I need the ability for the client to upload digital graphic files that we would be printing when they buy display hardware WITH graphics. So, when the client closes a transaction that includes selling him the graphics for the display hardware, I want to have the purchase transaction include the steps necessary for uploading the graphic file(s) needed for the order following the verification and completion of payment.

I work with a number of offset print vendors who provide this in their order process but I know they are using highly customized backend solutions that are beyond the scope of what we need.

PS, we are Mac based and create web site on Dreamweaver.