If you look at the item in 'orders' and then 'view payment details' for any sold item, you will see a breakdown of payments received and any deductions (BPF, promotions etc.).
If it's the speakers, your buyer paid nearly £20 for Royal Mail Special Delivery.
That service has a weight limit of 20kg and maximum parcel dimensions 61cm x 46cm x 46cm.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)