You could get three records sent Special Delivery for around about £6, so Second Class is just a joke.
I recently bought an item off somebody on eBay and was charged £3.75 for postage - when the item arrived it only had £1.90 on the label - so I queried the charge.
The seller sent me a snotty email saying that I had to pay for the packaging (all of which bar the packing tape happened to be recycled), their time packing and the 45 minutes it takes them to walk to the nearest Post Office, stating that these were all acceptable charges with eBays rules...
I don't mind sellers 'rounding up' but charging practically double is taking a liberty.
I left them negative feedback - stating that although the item was as described, their postage charges were ramped up and to avoid them because of this.
I think if they are unwilling to cancel the transactions, or combine at a reasonable cost, then this is your only option.