So we offer our clients a small discount to pay via direct debit which is automated. The price is a bit higher to pay manually. We caveat late payment fees on our invoice Ts and Cs, which are outlined here: AJRB Services | Support Center
This works well for us. We do however give a bit of leeway to clients. We give them the option to inform us (at least) 48 hours before the invoice due date if they’ll have trouble paying, then we waive any late payment fees and extend the invoice date.
Suspension/termination of services is the last thing I do as from experience it tends to result in them leaving, which equals less revenue than a late payment. That said, a discussion has to be had when it’s a regular occurrence. The client should respect that you providing a service incurs cost to you that you have to cover with the revenue of invoices.
The last measure we have done before, but rarely do, is send a letter with tracking. I’ve had a client before claim that they never received the emails. So I sent a letter and they signed for it then claimed they never received the letter, so I sent them a screenshot of their signature where they signed for the letter. They paid the same day!