You guys crack me up with your hilarious replies

Brilliant humour
Now to the main problem. "I'm a bit strapped." which is one of the many reasons we tradesmen hear for not getting paid.
But it's funny how you never hear anyone say that in Sainburys, or in the BP garage or in town on a Saturday night.
In short they have got money. They are just choosing not to include you in their expenditure. A good way to check is to have a look in the wheelie bin. If there's a pile of supermarket packaging rubbish in it then there is disposable income - but not for you. Is that fair?
I can understand how hard it must be to fall into a situation of debt with such a long-standing customer. So do what you can to save them. Ask them when they will have some money and if it's Thursday night for instance then make a special trip back on Thursday night.
If the problem persists then you've got to be tough. Avo is dead right. My motto is "No Doshie, no Washie" (taken from the TV show as you can probably guess). Two cleans past due + Two friendly requests for payment = Find someone more stupid than me. Supermarkets don't wait two seconds for payment. Why should we wait two plus months?