Slikone,
AmexBlue only compares the final purchase price of the item you are trying to PM. They don't include tax, shipping, or MIR's (they do include store coupons because they affect the purchase price shown on the receipt).
Here's an example. If item X is advertised at a B&M for $100 and you buy it there, then your total cost would be around $105 - $110 depending on your states tax. If you then do some searching and find this item online for $80, contact AmexBlue with the information and they will give you a check for $20 (so long as you bought item X w/ the AmexBlue card). They will still give you that $20 check even if item X came with a $100 MIR because that rebate does not affect the advertised purchase price.
My advice on this issue has always been to shop online for the items you want. Look for places that include free shipping and no tax (don't worry about the actual item cost yet). Purchase the items w/ the AmexBlue card then search around for the absolute lowest advertised price on the items you've purchased (you have 60 days to wait for prices to fall). Send the info to Amex and viola, you just got the lowest possible price without paying tax or shipping. And the deal gets better if you can find items w/ MIR's.
Happy shopping.