I guess this is being too picky - but I sent in our form I-751 yesterday via express mail. I enclosed a check of $275 and in the memo wrote "A# - Fee for I-751" and didn't have enough space to write "and biometric services". I guess this should be fine and they probably should not charge me for the biometric services when we have our date assigned right?

Don't worry, they know that the $275 check send along the I751 includes the $70 biometric fee.
The only reason I included my A# and description in the memo field on the check is for the unlikely occassion, should my checks be disconnected from my application before being processed (e.g. stack falling over in the mailroom and my check being burried with others). Then they could easily see where the check goes to. Other then that the memo field is more for your information only.