Public mobile is pretty reasonable about "honest mistakes" with add on purchases. Explain your error and they will likely credit your account. You can then use the credit for a different add on purchase or to help pay for your next renewal. Follow @pmbc 's instructions on how to contact customer support.
@Petemacw If you just purchased it, it's worth asking CS Agent if they can credit it back to your PM account. Then use that to purchase the data addon.
You can contact an agent using the chatbot. Type "agent" and click contact us to go through the wizard.
If for some reason they don't respond to the ticket, you can try to send an email directly to the CS Agent here.
Here is a help article about how to contact the CS Agent.