If I want to set flat rate pricing by weight, can I do it myself in my cart?

Yes, as long as you ship at the DDU level.

For any DDP shipments, the Passport team will need to make rate adjustments due to live Duties and Taxes. Any rate adjustment requests typically take between 2-5 business days, so please make sure to make any adjustment requests in advance. Self service is on our product roadmap for 2021 - and we will let our customers know once this functionality is live.