From what I remember of being subject to the 21 day hold thing a long time ago, automatic release just about never works. Assuming you're selling on eBay, here's what you have to do: send everything Recorded Delivery and upload the tracking number to eBay. Check the Royal Mail website and as soon as it says the item has been delivered ring Paypal with the tracking number and tell them to release the money. Stay on the phone while they do it.
Or, as soon as you see positive feedback from the person you sold to, ring Paypal. This means it's worth sending a message to customers explaining (nicely) that you won't be paid until they leave feedback.
When this was happening to us, I was ringing them once a day with a list of transactions to be released and I kept a spreadsheet with sale date, ship date, tracking number and delivered date.
