Hi,
You can use the "contact button". It could be use to send a message to the current vendor or contact the main vendor (the admin).
About Paypal, the value is not stored in a custom field but directly in the vendor parameters. So the best is to add an option to set the option as required.
We are finalizing the next version of HikaMarket (with new discount features), I think we could see to include such option in time.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.