We do a lot of businesses this way with 4-5 customers. The average order is about $60-70 with a lot of $100-$300 orders for company meetings and training sessions. The largest of them does about $6000 a year, the second and third largest are both about $2000 a year.
We bring paper plates and plastic, napkins and catering size salads when they want to feed a bunch of folks.
If we have problems getting paid we drop the program with that company. I have only had to do that once.
Our POS accomodates charge sales (for cash reconcilliation) and we keep a log book of charges, (who, what company, what department, date, order number, amount, order taker, driver and date paid)
Once a month I sit down and produce invoices. I use an invoice form I made up years ago on MS Word and I fax the invoices. Since I invoice the same people a lot, I just save the document and next time change out the order details.