Ok, you are still a wee bit confused

You would be paying interest because you are not at a zero balance with that card. Someone who is would not be, IF they paid it off within the confines of the contract agreement every month. One minute late and yes they would be paying interest. Every card has different rules, but most have a 10-20 day time span for you to pay off the card without getting extra interest added.
Anthea, you do NOT need that card, follow the plan and by this time next year you won’t believe you ever gave money away to credit card companies like you are right now.
Also, once you get going on a zero based budget and cash envelopes you won’t be having close calls with PSO any more.
Have you talked any to C about this? Maybe he can explain it better than I am.