Can’t pay expiring plan

Description

Hi I’m looking for advice or re-assurance really. I have 2 plans, one that expires in 20 months and one that is expiring on the 10th of July for £175 which I would like to pay off in full. Now I looked on the website and realised you can’t select an individual plan to pay off. I decided to phone PayPal and make a payment that way so it would go to the correct place and pay that plan off.
I asked the advisor if I could pay off the plan over the phone as I have the money in my account and would like to avoid interest. She told me that she couldn’t do this, if she applied a payment now the system would apply it to my 20 month plan. She goes on to tell me that if I let it expire past the 10th and wait until my next billing cycle on the 21st that any payment I made would then be added to the shorter 4 month plan I was trying to pay off. But won’t I occur interest ? I asked and she said NO you have 16 days from your next billing cycle on the 21st. This makes no sense to me and I’m worried I might have been lied to.
Now I really don’t want to occur interest on this, I’ve gone back in to my statements and looked at the 4 month plan. It clearly states this at the bottom..
If you do not pay your purchase off by 10 Jul, 2020, we will begin to apply interest of 19.9% on any remaining balance.
Can anybody advise me on this? Anybody had this issue?