Hi,
Can you please provide us the screenshot of your paypal plugin configuration ?
I thought that your cart was empty in your HikaShop store but it's not the case, the message you got is a paypal message.
I was correctly redirected to paypal and the cart was not empty in HikaShop ; when I went back in your website, the product was still there.
So I think the issue is related to your paypal configuration.
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.