I wonder if the mom is thinking the letter is about payment and it would so look good on her if she didn't read it and did show up on Monday. I am guessing with asking for the playpen for the weekend she wasn't planning on coming back so you beat her to it.
Hope your daughter is feeling better. Enjoy next week and check out the window for all arrivals and just don't answer the door if it is her. If she calls or messages just remind her of the letter in the diaper bag.