Commercial Late Fee calculations

[Joshua Kaufman]

I'm having an issue with rent collection from a commercial tenant. I offer an online payment portal to submit rent payments via ACH or CC - they "couldn't figure out how to use it" so I set us up to accept Zelle payments. To make things as easy as possible, I send a monthly Zelle request about a week before the rent is due. I'm still finding myself chasing them each month. 

The first few times, I didn't charge a late fee. That didn't help so now I'm looking to charge the late fee and interest as allowed by my lease. The least states:

"Tenant shall pay a "late charge" not in excess of five (5%) percent of the amount in arrears for each month the arrears remain unpaid if any Rent is paid more than ten (10) days after the due date thereof, to cover the extra expense involved in handling delinquent payments. In addition, all Rent payments hereunder not made within five (5) days of the due date shall bear interest at the rate of one and one-half percent (1-1/2%) per month"

The monthly rent is $1550 so the late fee is easy - I've set the account to charge 5% if the rent isn't received within the 10 days stated ($77.50). The interest calculations is where I'm having some difficulty. Is it a straight 1.5% on the monthly rent after 5 days ($23.25)? Or is it calculated daily and the base rent increases after 10 days when the late fee hits the account?

If it's the latter, is there an online calculator that can help?