Once you are enrolled(free), no annual fee. It's per-use rebate.
Say, you tow from home to garage, spent $90CAD and mail receipt to them and got $75US rebate in the mail.
Say, you are in Vancouver, you lock key inside car, call the toll free number, someone came and got they key back and spent $70CAD. Sent receipt to them and got $75US rebate in the mail.
Let say again, you are in Florida, need tow, and spent $125US. Send receipt to them and got $75US rebate.
It's a rebate type of usage, pay first and rebate later. If no use, no membership fee. Membership is 1-year after $25 spent with Onion.