From what I understand based on Next Pay and your posts I believe what has happened is:
You ordered online, using your usual Next account, and hence purchasing the sideboard with your Next Pay account. All online orders using your Next Pay account will be added to the account.
You didn't have sufficient available balance available on your Next Pay account to to order the sideboard, so we're instructed to make a PayPal payment.
You believed this payment was directly for the sideboard, however as you ordered via the next pay account it's just a payment on the account, not specifically to that item.
Then when refunded it was applied to your account.
On Next Pay all payments are off the account balance not the individual items. As this sideboard was ordered onto your Next Pay account rather than a non Next Pay account, the credit is credited back to the next pay account, as the PayPal payment actually paid the account not for the item.
How this stands legally, I am not sure. I am also not sure if the terms and checkout make it clear enough, from a legal standpoint, that this would happen.
I would contact and ask again if the payment can be applied back to Pay Pal instead. You may not get anywhere but you may. Be explicit in a concise manner that you intended to purchase the sideboard as a standalone purchase, not add it to the account.
The terms on the site are quite confusing, as they cover Next Pay account transactions and standalone strands too!, hence why some terms say refunds will be applied to Next Pay balance (for orders made on account) and others say refund go to payment method (for orders made via an account which is not next pay).