With payments received for tangible goods Paypal would request valid tracking information confirming the funds have been delivered to the buyer. As they can validate the tracking information online, once it is delivered they can release the funds. As you have received the funds for a service no tracking number is applicable and they would be unable to release these funds any sooner than the 21 days.
The buyer may have the option on their account to mark the transaction as delivery/satisfied. If the buyer confirms this by clicking on the "Confirm Receipt" button, the funds may be released to you sooner. This is not always guaranteed.
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