I recently went through the process of buying a mattress and wanted to share my experience.
Short summary: sleeptrain.com "LIVE CHAT" can do pricematching. I pricematched a mattress/box spring that sleeptrain.com wanted $950+tax to us-mattress.com and got it for $639 all inclusive! And they delivered the mattress the same day.
So price matching online retailers works at Sleep Train!