"We have already done the research and the benchmark amount is for purchases $2,500 or less. But with that said, if you have multiple transactions from a supplier every month and each of those transactions is less than $2,500, would it be better to purhcase those products or sevices via the PO process or use the One Card program?"
Companies typically set two types of limits on an employee p-card - one is a cap on how much can be spent at any one use; the other is a limit as to how much can be spent on the card in a month.
The former refers to one-time purchases and the best p-card practice is that anything below that threshold should be bought with a card rather than via a PO and invoice process. Therefore if you have multiple purchases from a single vendor that are each less than the one-time purchase limit, employees should be using the p-card for those. The aggregate spend comes in to AP as a single monthly statement whose total, will of course, exceed the one-time-spend limit. But this eliminates the time and expense of PO and invoice processing, while maintaining controls through the card program.
The exception to this would be if your company's PO policy governs certain types of spend that may fall below the p-card spend threshold. In that case, that purchase should go through the PO invoice process. P-card programs typically include blocking capabilities for certain merchant types, which you may employ to ensure that PO spend does not end up on a p-card instead.