Keep things simple. I believe offering to matte is a service that will pay dividends. I only offer white and black for matte colors and I only offer one frame style if a buyer wants a frame. Most successful photographers offer these services. The majority of my buyers take me up on providing matting and framing, but they do have the option of purchasing a non-matted print if they choose. I don't make much money, if any, by matting or framing. I look at the additional effort as making for a more user-friendly business. An equally important benefit to doing one's own matting and framing is for doing shows. Paying someone else to matte and frame is not cost effective. Anyone seriously interested in selling photography must do shows. Having a website alone will generate little if any business.
It is a good practice to sell prints that can be matted to standard frame sizes. The 2:3 aspect ratio is not user friendly in this respect. I'll give you my work around for what it is worth. As for print sizes, I only offer three: 10x15, 15x22.5, and 20x30 (These are the actual print sizes and does not include borders and I do explain they are not necessarily exact sizes but will be close as I will do compositional cropping on some work). As for finished matte sizes, I offer 16x20, 22x28, and 24x36. I would never crop a photo to fit a standard frame or matte size, and my mattes are not equal in on all four sides, but I do not believe this detracts from the presentation. My largest prints have "under-sized mattes" if one goes by recommended standards, but I find larger prints display just fine with substandard matte borders.
Best of luck,
Greg