When you archive a published campaign (because you stopped it early, or it finished with budget left over), the unspent amount comes back as an ad credit. That includes any part you paid with an ad credit, so there's no need to email support to get it added back. Refund credits expire after 6 months.
Who gets the credit
If the campaign owner is an agent, the credit goes to them. Agents only see ad credits assigned to them.
Otherwise the credit belongs to the workspace. Workspace admins can see and use every ad credit in the workspace.
Using an ad credit
Advanced Setup: in the Budget & Schedule step, go to the Ad Credits section and use Select your ad credit.
Smart setup: in the Budget & Billing step, go to the Ad Credits section and use Select your ad credit.
Done for you, and Ready to run cards for your listings: on the review screen, click Change card or ad credit.
You can apply one ad credit per campaign. It comes off the amount you're invoiced.