Hi @artpicsandguitars .
1. Yes, you can enter different dimensions before you purchase the shipping label.
a. If you do this, and if your box size increases from your original estimate, it may mean you're paying more for shipping than you charged the buyer.
2. As long as your items will all be shipped in a box that is less than 1 cu ft and none of your items are shipping in a box longer than 22", then what you can do is enter 12x12x12 on the listing page. That means you will not risk undercharging buyers for USPS shipping services. When you purchase the shipping label you replace 12x12x12 with actual dimensions.
a. How to calculate volume? L x W x H <--- as long as that number is below 1728 you're below a cu ft.
3. As a new seller, eBay defaults your account to charge buyers the carrier discounted rate.
A question you need to ask yourself is is whether you want to charge buyers retail rates or discounted rates. Keep in mind that you have access to discounted rates when you purchase labels on eBay regardless of how much you charge the buyer.
The setting to offer buyers discounted rates can be found on your shipping preferences page under the carrier specific discounts setting: https://www.ebay.com/ship/prf
Since eBay charges a fee on total cost buyer pays (item + ship + tax), by charging discounted rates you are losing money on every single package. If you want to make up for that you can add a handling fee or increase your item costs - or simply charge retail rates for a built in cushion instead of passing on the discounts.
