I bought a B-square mount off ebay last year for about $5-7 cheaper than I could get in the store (after I paid shipping) and it's worked fine. Nice solid mount. However, attaching the mount to your 870 will mar the finish.
Also bought a red-dot scope a little bit cheaper. Not a name brand, but it has worked fine so far.
Also bought a red-dot scope a little bit cheaper. Not a name brand, but it has worked fine so far.