If your supplier needs pre-payment or a deposit on a purchase order, ask for an invoice from the supplier that you can submit to Accounting.
For Accounting's ability to expedite payment, the invoice should reflect the percentage of the discount on all line items, rather than one bulk sum so that the pre-payment or discount amounts line up to specific line items on your purchase order.
When you have the invoice, submit it to InvoicesOnly@bfs.ucsb.edu and ensure that it references your full "GW" purchase order number.
Make sure to also cost receive on your purchase order for the amount of the pre-payment/deposit invoice's grand total, so that you are approving payment to be released against your invoice.