la9 - I am pleased to report that the second line of code changes you provided to alter line 290 of payment/paypal.php has RESOLVED the issue and the guest checkout is now working fine. Looks like adding the (int) code to both lines 275 and 290 will sort this issue quite easily in future releases.

Thank you so much for your indulgence in getting this working for me.. One of the downsides of Zen Cart (for me) was always the over long checkout process. Last time I used Zen it was the COWOA checkout, but this OPC guest checkout was the final obstacle to me returning to Zen full time. All sorted now - thank you.!!