Ground and priority shipping questions.

I’m selling the same sized object each auction. Think the size of a Funko pop.

1. I set up my shipping for ground to make it cheaper for the customer but if I want to send it Priority instead can I bring the package to the post office and just pay the extra amount but use the same shipping label from whatnot so whatnot knows I shipped it and I get paid for delivery?

2. Do we always have to use the whatnot label or can we manually add tracking like eBay?

3. I did bundle shipping for 4 items. 1 item I have the perfect ground box, 2 items doesn’t really fit in any box well. 3 items fits perfectly in a priority box. Is there a way to adjust shipping based on how many they buy? There’s no way of knowing prior as to what quantities I’m working with.

Thank you in advance!!!