Music Store Need To Allow for Combined Shipping on Multple items in one box

Hello,
Thank you in advance for any help you can offer. I am building a music store which sells 45s, LPs CDs, posters, etc. If someone orders more than one item, I need to offer that person a combined shipping rate discount to reflect the price that the post office charges for the items I am shipping.

I don't think the USPS module will be able to know how much to charge if the module doesn't know how I combine the items together in shipping/packing (one box vs 3 individual boxes). Please let me know if I am missing something about the USPS modules which account for different shipping package situations.

Is there a way to manually calculate combined items shipping discounts? If I know that someone bought 3 cds and 3 LPs and it will cost $18 to ship, can I manually offset the order after the items have been purchased? Is there a way to adjust the order's shipping price after the items have been purchased but before the items have shipped?

Looking forward to a response.
Thank you. Mitch :clap:

Actually, Mitch, I can vouch for the fact that the USPS module DOES assume that you are combining products into a single box. I know this because my client needs to figure out how to enforce each item in its own individual box, and am still searching for a way to do so.