You can developp your own, print solution, technicaly, rpg is enough. But in this case you need strong programmer. IE TCO will reach vendor soft.
IMO you have a better solution using electronic exchange : e-mail you PO’s, and pay your vendors by electronic payment. TCO = 0. you can transform your PO in flat file or html page or pdf file (look at MMAIL), you can email with native SNDDST or with the free MMAIL (from easy400.com or ibm.com) ; and you can send payment order to youbank with the soft your bank will offers to you.
And you will save trees.