I would expect, a supplier may reject, if there are grounds, such as non-payment. I know this is so with suppliers with existing payment terms contracts between them and their regular customers. It is usually stated there that future POs may already be rejected if there is a non-payment.

It's not certain kung tuloy silang mag demanda, either way, if they could win over you or not, it's an abala for both you and them if ituloy. I know you've tried already, but try further talks with them as possible to resolve payments for previous delivery, as a requirement for serving their PO. You may also need to give some form of assurance to them that your trucker will now only dump the load out after it is inspected (and D.R. signed?).

Hope things work out.