We pay in arrears and i pay monthly, never get an invoice and everyone is different some pay weekly etc. BUT at the moment i am a month ahead of myself!! Good eh!!
I always pay 4 weeks in advance and as my YO lives opposite I pop it over sometime the week its due -usually before, sometimes after. Half the time she's forgotton its been 4 weeks already!
We pay our stable/grazing bill by standing order, and our livery services bill weekly or monthly depending on our arrangement with YM. I always try to pay immediately I get the bill. I do think this is only polite. After all, she never says, 'I will do your horse for you, but you may have to wait a couple of days'.!
I pay on online banking before it's due, I never get an invoice.. if I do it aways say I owe them and get told to ignore it (because I don't owe them) and I am a month in hand.
I pay weekly but am a week in advance. Get invoice on Friday, usually paid by the Monday, but sometimes it's late, sometimes it's early! I'm very random, as pay in cash so it's when I remember to go to the bleeding cash point.