This is the bit that Paypal don’t tell you about.
An eCheque payment occurs where you have funded your payment with a debit bank account, and you have no Credit Card on file with Paypal as a ‘back up funding source’.
It also occurs if you manually select the "eCheque" option, even though the options for "Instant" and "Credit Card" payments are also offered.
This type of payment is literally a ‘cheque’, and as such, is subject to a clearance period. Like a cheque, while your bank account may be debited within 24 hours, the payment is not actually transferred to the seller for anywhere between 4 and 10 days, and in some cases even longer.
Unfortunately, the buyer gets an email from Paypal that suggests they had made an instant payment.
This leads to much confusion and angst!
In my experience so far, the shortest time an eCheque has cleared for me as a seller, is 5 days, and the longest has been 10 days.