ACH payments are free. There is a flat fee to pay by debit card ($9.95 for rent and $4.95 for other rent-related charges) and a percentage fee of 2.95% fee when using a credit card.
If you live in Colorado, you can only choose credit or ACH as your preferred payment method. If you're currently using a debit card as your preferred payment method, you can continue to submit rent payments without any fees.