I'd be very careful with your proposal, Jaime. Don't do anything with it before your bank checks with legal counsel. You have lease contracts with these customers, and it's unlikely that your contract provides for adding new fees. At best, I think, you might be able to add the fee as leases come up for their (presumably annual) renewal.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8