Seller Fulfilled Prime and Royal Mail - OBA requirement

Hey, I've been searching for an answer for weeks and just went through to customer support to get absolutely no helpful information at all.

I had been purchasing shipping through Amazon for many orders, specifically using Royal Mail and didn't have a single issue. I enrolled in the prime trial however, and I've lost access to everything but Evri as a carrier.

I've searched everywhere I can for answers, but as far as I'm aware I need to link a OBA account now to continue purchasing shipping through Amazon since starting the trial. I've been selling for years and have only just made a return, and due to not being able to sell next day on Amazon (my items are greetings cards and gifts) I don't currently qualify for an OBA account.

This means I need to purchase my labels through the Royal Mail website, but a qualification for prime requires me to buy the shipping directly on Amazon.

I've read multiple articles about people being able to buy shipping without OBA, but everything I try doesn't give me these options back. I thought I would ask here and customer support told me to just link my account even though I stated I couldn't, and even told me to just buy Royal Mail shipping after stating it's impossible. I've even read multiple discussions where people have responded stating that they sometimes have the option, but I can't get this to work at any time of the day, with any shipping templates. It just states I haven't linked my account, and tries to force OBA.

I have Amazon integrated on Click and Drop, I just can't get access to the details needed from Amazon due to not being able to use OBA.

Any help would be appreciated to the moon and back!