Hi there @sophearot!
Don here from Shopify.
Thanks for taking the time to share your query with us here in the Shopify Community!
If you're looking for a way to allow your customers to add a file upload in your store then you can check out the range of upload apps available in our app store.
As with all third-party apps in our store, you can reach out to the developers directly with any support queries you have by using the contact details listed there on the app store page.
If you'd prefer to get a custom solution created for your store you could consider hiring an Expert developer from our Marketplace to put some custom functionality in place for you.
Would this be something you need to have in place for many of your orders do you think, or is it a marginal amount of trade?
Do you know what else you need to work on next here in terms of your store or business?
In any case, let us know here how we can help out!