I am confused as to why a small dollar PO would be an issue but there are several alternatives.
PCards - these are really handy for small purchases, you lose some control but you gain a bunch of flexibility.
PO against contract - satisfy your legal requirements in a Master Services Agreement or services contract and then PO release against that contract.
blanket PO. establish a PO to be invoice against. Again you have an agreement in place but the BPO gives you the flexibility of not having to create a separate PO for each action. Great for Facilities.