Selling custom product (data imported from url)

Hi all, I am setting up a webstore. Most of the products are physical and easy to implement with SS. But one custom product is a headache.
Is this following thing in any way possible with SS? I dont mind using 3rd party solutions and API:s, as long as it works.

-Another site has a link to my site, with the "product" (.stl) appended to the .url

-The .stl file fetched(correct term?) from the other .url is both used in my site to show how the product will look like (I already have an API) and in some way sent to me during the purchase, so that I know which "file" they have purchased.
-I think saving the data file to SS directly is impossible, so if I want to save the file, a 3rd party solution may have to be used here. Another option might be to just fetch the data always from the other site, and just send me the .url of the purchased product during the checkout. 

I have done lots of programming, but am a complete noob in website design. So for me this is both a general problem in html/js programming and how to implement it into SS -site.

Any help is appreciated, I would be glad to just be pointed to a proper direction. Even if its away from SS.