Can Margin enter a monthly bill for me?

Yes. In Reference → Recurring, set up the bill once: company, cost head, vendor, amount, how it's paid, how often (monthly, quarterly or yearly) and on which day. Margin makes a draft each time. Tick Provide at month-end if the bill is late and, if the bill isn't in by the 5th of the next month, its cost is booked to the right month anyway, and reversed when the bill arrives.