[tibordp]

Given that we are talking about A4 papers and grams, I'd bet this wasn't in the US.

In Europe, the typical flat rate is up to 100g for standard letters. And that's 20 sheets, which is not a particularly unusual letter to send.

[prmoustache]

But 20 sheets do not fit in a regular DL or C5 envelope so you already have an hint that you may check the limits, you usually send them in a reinforced C4 enveloppe.