Ah dang that sounds tough.  

Should it be on your invoice?   

And, yes, most tenants pay via QuickBooks Online invoices, so accounting for the ACH delay is another vote for moving the rent due date. 

Do people have luck with having a relationship with a local handyman who can just invoice you hourly/per job, or do unexpected maintenance calls really happen that frequently? 

Send them an invoice for the fee (via email is easier) with a deadline. 

Are all my invoices uploaded to my owner portal?

Finding serious buyers is pretty easy when you have great deals, and dang near impossible if you don't.

Ten units is a bit of a nebulous thumb rule, but quick books allows you to fully run a business (ex: invoicing, payroll) and most people when they get to that scale are moving beyond the basic SFH rental needs and have more advanced requirements.....

Josh McCullough, do you invoice the tenant or send him a BILL?

The sellers have to sign off (at least in CO) to return the EM, and they're pretty dang unmotivated to do so after the property goes to sale.