Can my form calculate items based on the quantity being shipped?

1.       Shipping: I am going to use the Fixed Rate envelopes and boxes through the USPO

Can the form calculate the exact box or boxes needed based on the quantity of each item they are shipping?

Yes, can try to use conditional logic for this purpose. Please check the guides below if they suit your purpose.

https://www.jotform.com/help/57-Conditional-Logic-for-Online-Forms

https://www.jotform.com/help/268-How-to-Insert-Text-or-Calculation-into-a-Field-Using-Conditional-Logic