Regardless of who is in the right, their letter was more professional and polite than this OP response. The author is mistaken to think this is a pushy or aggressive letter, compared to an actual pushy or aggressive legal letter.

PS: In the U.S. (if not the rest of the English speaking world?), if you want to use an honorific for a woman but don't know which she uses, always use "Ms.", never ever "Miss." "Miss" implies a woman is unmarried; "Mrs." implies that she is married and the surname is her husband's; "Ms.", appropriately for professional communication, makes no assumptions about her marital status.