Tracking confusion

I ordered from sea of seeds on march 1. I was given a tracking number with royal mail. it said the package wsd given to usps. How do I track it from there? Royal mail have me a 13 digit tracking number. Usps wants a10 digit. Sorry if I'm stupid.

It'll accept it if it's 13 or 10 or 100000000000 as long as it was a valid tracking number. If you copy&pasted and it didn't work read Q&A.

Worked like a champ. Processed 2 days ago so hopefully when I get home from work tonight.....