What do you think?

[Lee-Bee]

Does she walk in during regular hours, or knock and wait? I see no reason for her to assume that because she is late she should not longer walk in if that is what she usually does.

If this is the case then the real issue is that she is late and that her payments have been late. Discuss those matters with her.

if you don't make it known (charge late fees if needed) then the issue will remain unsolved and she will continue with being late and having late payments (or near late if you remind her).

Stop emailing just before payment is due. It's not your job to do a last minute reminder. Let her be late, charge the late fee and she will take it upon herself to change her behavior (because she has a reason too).