USPS just had their annual price increase, so the new price is $18.90. Should the rules be edited to reflect this?
And it should be 'an' before the price, not 'a'. This is an $18.90 flat rate box.
Or if you really liked the 'a', you could rearrange the sentence a bit. This is a Large USPS Flat Rate box, for which the postage cost is $18.90.