And...
The £82 invoice did get cancelled. (Yay!)
The invoice for March arrived, and as expected, it's for £55, so we've lost our previous discount. After paying that, we have just under £100 in our PayPal donations account, so we're OK for a bit. But our yearly costs for the future look like this:
£660 (12 months of Nimbus hosting at £55/month)
£112 (1 year XenForo forum software license plus domain name registration, likely due in May)
====
£772 total for 1 year
That works out to just under £65 per month, so we've raised our monthly donation target from £45 to £65.