I had the same problem with Lowes and Walmart when I ran a small engine shop. It took forever to get paid but they wanted the equipment back quickly. I had to stop delivering fixed equipment till they got their accounts paid to date a few times.... I had their customers equipment and the customers pushed them to pay me.
Shopbot required full payment in advance before they would even start to build my machine... they didn't have any problem telling me that if I didn't pay the next person would just get the machine and I could wait.