Tenant constantly late paying back rent

[Keno Powers]

Hi Everyone,

I have a tenant who moved out from a Maryland rental I used to have. I gave her notice in February of 2024 because she wasn't able to pay full rent. She moved out in March. I sold my property in April of this year. My tenant owed me about $1,500 in back rent and damages. She has a balance of $723 left. It has been like pulling teeth to get money from her. If I don't remind her to pay, she won't. I plan on sending her a payment agreement so she'll pay every month on time without me having to remind her every month. (Is that wishful thinking?) 

As an incentive for her to pay on time, can I charge her a late fee after 5 days from the due date and can I charge her for every day she's late? The other question I have is can I charge her interest as another option? Does she have to sign the payment agreement for it to be enforceable?

Thank you in advance you any help or wise words of wisdom?