Tenants are chronic late-payers

[Tori Breitling]

First time landlord here, so obviously I need to do better pre-screening. What I'm wondering is what I should do now. Tenants have been in only for four months, and have paid late two of those times. I don't think it's really a money issue, but more of an inattention issue. They are business owners trying to keep their business afloat, and so are full of excuses about why this and that. I actually like them, and they are taking care of the house, and I'm sure they don't want to move. But how do I get them to pay on time? Trying to get them to pay me is time-consuming and stressful. They seem to think the late fees are a minor nuisance. Other than flat-out serving them notice the next time it happens, refusing payment, and moving forward with eviction, are there any other strategies I might consider?