Great idea! Framing is an incredible hassle for exactly the reasons you describe. Would be cool to partner with small artists to pass out discount codes on physical cards they could include with rolled & shipped art.

For the future -- provide exact specs on the acrylic you're using to front the pieces (and consider offering museum glass in the future). UV exposure is critical depending on the placement of art and, perhaps ironically, particularly for the more inexpensive pieces people may be wanting to frame with your service that may have been printed using materials more prone to fading/etc.

Also, when you eventually offer additional matting colors/textures, put together a sample pack (using the cut scraps) that can be mailed out. Mats, particularly colored mats, look very different when actually adjacent to a piece of artwork than viewed on a screen (much more so than most framing material).