Answer: Making an accommodation like this without a writing is an invitation to problems. You can easily enter into an addendum to the Rental Agreement saying that "Rent will be paid by the 11thof the month, and late after the 15th"(i.e. 4 days per statute).
Under this scenario, a 72-hour Notice could issue no earlier than the 18th(seven days from first date rent is due, per statute). If the Rental Agreement provides that there will be a fine of $X for a bounced check and a late fee of $X, then you're OK to do so.
As to whether you created a problem for yourself by allowing the late payments, I would say "No" so long as you have it in writing. Without a writing there is too much chance for argument as to when rent is late and when a late fee can issue.
MHCO does not have a form for this, but could. I will check.
Lastly, to your question about insisting on a certified check or money order, I would like to see it in the Rules or Rental Agreement, so it is enforceable. If it is not in your rules, you can still tell the client that henceforth, he or she will have to pay the rent by certified check or money order, but you could not enforce it, if they refused.
The rule can simply say: "In the event a Resident's rent check bounces on more than one occasion within any twelve month period, all further payments for the following 12 months shall be paid by certified check or money order."
But this would have to be an amendment to your current rules, following ORS 90.610.
