Please note that we need to upgrade the Trello premium plan for one user login, and I have activated the trial. However, the billing shows $12.50 per month, but the yearly billing shows $1319.89. Why does it show a higher value?
$12.50 is the monthly cost per user. However, upgrades apply to the entire Workspace rather than an individual user; it's not possible to upgrade only a single member within the Workspace. Once you upgrade, the bill is automatically calculated taking into account all users. In this case, you have 11 billable users, so that annual total would be $1319.89.