For the USPS ... if it is $1.00 per set of 2 ... how is that Flat Rate? Or did you do a lot of customization?

On the other two ... what happens if I buy 11 sets? or 20 sets?

You might be able to start with Item Rate and re-calculate the quantity in sets of 10 for the rate charge where any remainder increments it to the next set of charges ...

something like adding beneath the $item_total_count in the module:
$item_total_count = round( ($item_total_count/10) +.5) , 0);

NOTE: I did not test the calculation ...