On checkout “Error Detected: Your shopping cart is empty.”

Hi,

I have WordPress v4.3.5, WP e-Commerce v3.11.3 installed on my site https://michellescardsandstamps.com.au, the site doesn’t have SSL and my problem though is that during checkout and redirecting to paypal it shows the error "Error Detected: Your shopping cart is empty." This does not happen always but sometimes, does this have to do with sessions/caching?

I’ve flushed my cache, flush the permalinks excluded the checkout page from caching but the problem still persists. Is there any other way to fix this issue?

Any help would be appreciated. Thank you.