Each domain using Pair Pro has a quota of 1 GB per mailbox. So a 1 mailbox plan has a quota of 1 GB, a 3 mailbox plan has a quota of 3 GB, and a 5 mailbox plan has a quota of 5 GB.
If the combined size of the mailboxes exceeds the quota, you be automatically upgraded to add an additional 5 GB of storage. Upgrades continue in 5 GB increments. The cost for a higher quota is $1 per month per 5 GB upgrade.
The upgrade cost will be charged on the last day of the month, and it is based on the quota used on that day.