When credits refill
Your monthly subscription credits reset to your plan's limit at the start of each billing cycle, and unused credits don't roll over. Yearly plans add a one-time bonus when you first upgrade. Top-up credits you buy are separate, they stack on top and never reset.
Your monthly credits reset each cycle
Every plan comes with a monthly credit allowance. At the start of each billing cycle, those subscription credits reset to your plan's full limit. Whatever you didn't use in the previous cycle does not carry over, so there's no benefit to hoarding them. If you process a lot of mail one month and a little the next, the next cycle still starts you fresh at your plan's limit.
If you upgrade mid-cycle, your larger monthly allowance applies right away, so you have more credits to work with for the rest of the cycle.
The yearly bonus is one time
Yearly plans add a one-time bonus the first time you upgrade to them: Plus adds 200, Pro adds 500, and Ultra adds 2,000. This bonus is granted once, not every cycle, and it sits on top of your monthly allowance. It's a head start for committing to a year, not a recurring extra.
Top-ups don't reset
Credits you buy as a one-time top-up pack are separate from your monthly allowance. They stack onto your balance and stay there until you use them. The monthly reset never touches purchased credits, so a top-up you don't finish this cycle is still waiting for you next cycle. Your balance in Settings under Credits shows your monthly limit and your purchased credits separately, so you can always see which is which.