Is there any known issue with Royal Mail Buy shipping services in the UK?

We are unable to generate labels now with an error that "Seller is not registered..."

please can anybody help me? We are new to Royal Mail business and we get our postage through buy shipping Royal Mail it’s been a month and I believe that I am being overcharged by Royal Mail. We have sent numerous amount of emails to them but getting automated responses can anybody tell me, once we get an invoice from Royal Mail, how can we check if we are being overcharged? I mean of course I look at my manifest and count how many tracked Royal Mail 24 I have used but then where am I comparing it to? I can’t find it on the Royal Mail OB a platform or is there a matching barcode or maybe a postcode from the Amazon seller account and also the Royal Mail account where I can match them and know my charges.. because we feel like we have been overcharged and we don’t know how to check each transaction how do other sellers do this?

How do I get Royal Mail International standard/priority 'Letter' through buy shipping? I don't use 24/48 tracked on my business acvcount so I don't have it linked and at present buy domestic second class letter through Amazons buy shipping.