Good afternoon, Motico.
(1) I MAY have an answer.
(2) Mr. Raymond Murphy in his Grammar in Use (for intermediate students) says:
SOMETIMES it is possible to say "if you would," especially when you ask
someone to do something in a formal way. His example:
I WOULD be very grateful if YOU WOULD send me your brochure and price list.
(As you know, "would" is the past of "will." In English, as you probably already know, the past is considered more polite. Hey! CAN I sit here? > Excuse me. COULD I sit here, please?)
(3) You want to know why your example uses "should" instead of
"would." I MAY have the answer.
(a) According to the rules, one should use "shall" for I or we:
I shall call you tomorrow; We shall be delighted to visit you.
(b) Most Americans ignore this rule: they use "will" for "I" and "we."
I will call you tomorrow; we will be delighted... .
(c) Therefore, your sentence is "PERFECT": I SHOULD be grateful if you WOULD ..../ I SHOULD greatly appreciate it if you WOULD ....
(Remember, however: Americans feel more comfortable with "would.")
Have a nice weekend!
P. S. Of course, you know that "should" is the past of "shall."