We do not attempt to match purchasers to a member number at checkout.
You can add Member ID’s as a reference point for your own purposes in the Member Info page, but it’s not something we would use at checkout.
It’s important we don’t slow down the payment process with unnecessary steps. It is much better to get money first and foremost, versus having to reply back and forth through unnecessary customer service emails prior to payment. Your members have undoubtably not memorized their Member ID or most likely, any clue they even have one.