Is there any way of paying for postage with funds being processed or available funds?
No, there isn't. Under eBay's new system, sellers now need to have sufficient funds to cover the cost of postage, until their cleared payments are available a few days later.
Alternatively, perhaps you could pay your p&p costs from a credit card, which would give you longer to pay.
If the postage is costing more than the items, I hope you're allowing for eBay's fees in the price you're charging? Remember that you will in effect be paying more than double the fees payable on the item itself. Ask yourself whether it's worth it? Bear in mind that if a buyer finds any reason to use eBay's money back guarantee, you will then have to refund the full purchase cost - and probably let the buyer keep the item rather than pay more than the item is worth to send them a return label. Don't forget that eBay now allows buyers to claim a full refund, with the seller even paying for the return, it an item arrives even one day beyond the latest estimated delivery date! Sellers really need to convince themselves that selling here is worth the risks.
