Brevet card, poly bag and note shouldn't exceed the weight limit for a normal letter. I'm certainly surprised that it exceeded volume as well. I've sent out several things in the past that would have weighed more and filled more space.
The last time I fell foul of this I was in an odd position of being both the recipient and sender. We were living in two different houses at the time and I needed to send my partner's ID card to her at the other address. I handed it over at the post office and asked them to stamp and send if for me. The card dropped through the door the next time I was there a few days later and beyond the "first class" deadline so I went over to the collection office. When I recognised the envelope I complained as it was the Royal Mail (through business relationship with the Post Office) that had assessed and priced the envelope. The person behind the desk rechecked the envelope and confirmed that it did fit and waived the surcharge, refunded the stamp price and apologised for the inconvenience. Mind you, at that particular collection office, I did find the staff to be very friendly and helpful, which isn't always the case.
Anyway, should you get hauled to the collection office and reckon they're pushing it, ask them to demonstrate the excess there and then. I'm not sure what the rights are in this position, but I reckon any customer has the right to request a review of an applied surcharge before determining whether it should be paid.