Your user ID is assigned to you by your work or school. It looks like ellen@contoso.onmicrosoft.com, ellen@contoso.edu, or ellen@contoso.com. This work or school account is the email address and account that you use to sign in to Office 365 for business for work or school.
A postage meter can be leased for an averages cost of $20 to $60 per month. That is the cost for the average sized postage scale, 30 pounds or less. Meters with larger scales will be pricier.
By the way, I passed by the Elephant & Castle today. With the grey skies and misting rain, I almost thought I was in London for a sec. FSM: I mail letters to penpals once a month and the stamp is always 84 cents - and I write long letters. Maybe it's the size envelope you're using as that will change the rate. Or maybe they're shipping your letters via the Queen Mary vs. airmail.