So you can do this a couple of ways, and you can figure out what is the best for you.

When entering in the Square Point of Sale app they will choose invoice as payment.  You can set your invoice date for the end of the month.   Just know that Square doesn't send out a statement they will send out every invoice.  So if the customer was in every day they will get 30 emails at the end of the month.  This may not be what you want.

Another way is to just send it the same day so they get the invoices throughout the month.  They can pay them as they come or at the end of the month.  Again Square doesn't send out Statements and you can NOT easily create one.  What I do is send out a generic email reminding them to make their payment.

Also, know that they have to click on every email and pay each invoice individually.

Now if you ask the customer to save their card on file then the invoices will automatically be charged when and the customer doesn't have to worry about it.  So if you have all the invoices come do at the end of the month they will all be charged that day if the customer allows the card to be saved on file.   Know that you will be paying the manual card entry fee.

Another option is to get physical gift cards.  If this is a regular thing you can get a customer card made.  Maybe a "Cool Coffee Club" card or whatever.   Physical gift cards have no processing fee and you can reload them.   So the customer loads the card with $50.00 at the beginning of the month (you will pay standard processing fee if they load it with a credit card.), then every time they come in you can swipe the "Gift Card" and it deducts it from there balance.  Now if you don't want them to have the card keep it in a file with there name on it after you load it.  The first time they use it you can link it to there name by using "Save Card on File".  Then all they have to do is give you there name and you can look up the account.  You want to keep the card on file so you can reload it.  Their receipt will tell them the balance left on their account after each transaction.

I hope this helps.