I too wrote "usted paga" and not "tu pagas". I have reported it. In fact is it not correct that "tu" is superfluous if one writes "pagas" whereas "usted" should be used to avoid ambiguity?
It accepted "Pagas catorce pesos."
It didn't for me...
How am I supposed to know that it's informal and not formal here??
You're not. All the "you"s should be accepted. They may not have been added to the database yet, so use the Report button.
Shouldn't paga usted be accepted?
Tu pagas catorce pesos