Hi @Screen Doctor,
I'm here to take care of your concerns.
For your first question, take a look at this article for the steps on how you can import your QuickBooks Self-Employed data: Switch from QuickBooks Self-Employed to QuickBooks Online.
If you have QBO Plus, you can use class tracking and assign it to the line item that shows the amount for postage. As for the report to use, it's the Sales by Class Detail.
Additional details on this process are in this article: Get started with class tracking in QuickBooks Online. It's a step by step guide on how you can utilize classes in QBO.
Have other questions in mind? Place them below, and I'll get back to you.