You’d think they’d be more on top of things. Another option is to set up payment through your bank’s online bill pay service. Most banks have then now and offer it as a free service. I pay our mortgage that way every month. Basically you set up the service with account numbers and payment address. Then you go in and pay them when you want with the amount you want. The bank then either sends them the payment either electrically or a check on your behalf (depending on what the payee allows). I’ve never had any issues, but just make sure you double check the account info when you enter it.
Not sure what day “end of the month” but if got a check in the mail today, I wonder if it would get there on time. Having said that, I agree with what Sharon said. I would pay the extra $10 to make sure your payment gets to them on time. Just this once. Late fees would likely be higher than that.
Hope you’ve got a lot of money saved. Since May 1st you will have another payment due.