Help am I in trouble?

Hello, I recently ordered from bullion by post, I thought that I'd be able to pay with my debit card or though pay pal but I paid with bank transfer. It says "You must initiate a bank transfer, deposit cash at Barclays or post us a cheque within 2 working days or your order may be cancelled. In the event of cancellation a fee of �25 plus a charge based on any adverse movement of the underlying metal price(s) may apply." So if I don't pay how will they charge me? I put my first name last name and address, no other information. I got an invoice letter. I want to buy the silver but I don't want to be charged even more for a bank transfer. So what should I do, I want to buy it but I don't know how bank transfers work and what to do with the invoice letter. I also don't want to be charged more for a bank transfer. please help, this isn't the most structured paragraph but I'm quite nervous and tired :(