You should figure out websites that have affiliate programs, force the requester to choose a website, and when you end the text message, include the website to add some validity. Including the well-known brand will seem like it comes from a more legitimate website than you (no offense).

For example:

"Hey Andy!Someone shopping on Zappos.com for you and wants to know your shoe size. Reply with your size (ex '9', '10.5')

Also, when you send back the sizes to the requester, you could include an affiliate link for the site they requested to help monetize.