I'm in France and these checks are US checks. I go to USA once per month to collect them (they are sent to a PO box) and deposit them. That's why I only do it once per month.

I'm actually unfamiliar with the deposit slip. My bank's ATM simply takes checks one by one and I use my credit card to log into my account to have them deposited into my account. Can you explain what deposit slips are?

I shall check with my bank, thanks for suggestion. When I opened my account they simply told me to do it at an ATM.