Buy Shipping Services does not support the use of different packages for separate parts of one product. This means that it is not possible to buy separate shipping with different labels and tracking numbers for individual parts or pieces of one product listing.
To buy shipping for separate products in one order (individual line items, not multiple parts of one product) buy shipping for the first item by including the quantity of each item in the package. Then, return to Order Details to buy shipping for the next item. Repeat the same for other items as required.
From your seller account, go to Orders, click on Manage Orders, and then click on Buy shipping to start the process.