You are an infrequent seller who has recently sold two high value lots.
If you look at 'payment holds' (see the link below) you will see that this is part of the terms and conditions you accept by selling on Ebay. Payments may be held for 30 days.
You must also consider that buyers can return items for up to 30 days and would have to be refunded in full, possibly leaving you in financial trouble.
If you let these buyers down, they can leave you negative feedback which, as you only have a few recent sales, would have a serious impact on your account.
Can you not arrange a short-term small overdraft, borrow from family or use a credit card to pay for postage?
https://www.ebay.co.uk/help/selling/getting-paid/getting-paid-items-youve-sold/payments-hold?id=4816...
"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)