Straightforward, but check the dependencies first.
BEFORE CANCELLING
Work through everywhere the address may be referenced:
Your domain's A record, and any subdomain records
Third-party whitelists: payment gateways, APIs, partner systems, corporate firewalls
Mail configuration, including SPF records naming the address Application settings that bind to or reference it Any documentation you have given clients or colleagues
Move each of these to the shared IP, or to whatever replaces it, before the address is reclaimed.
HOW TO CANCEL
Services > My Services > click the service > Addons, request cancellation, choosing end of billing period. Or open a ticket.
WHAT HAPPENS
At the end of the period the address is removed from your account and returned to the pool for reassignment. It will not be held for you, and you cannot get the same address back later.
AFTER CANCELLATION
Confirm your site still resolves and loads on the shared address. Confirm email still sends and is delivered. Confirm any third-party integration still works.
Check these the same day, not a week later when the cause is less obvious.
