I am having a problem for which I haven't been able to find a solution. Help would be GREATLY APPRECIATED please!

I am just newly setting up my shop and have most things completed and am now loading in the stock.

In testing a purchase, every time it needs to go across to secure mode, an error message is displayed saying :

'This page contains both secure and non-secure items.
Do you want to display the non-secure items?'

I just can't discover what is causing this so that I can remedy it??

Here is my store:

http://www.natureswonderland.biz/Shop/index

Thanks heaps for any help :)
Sha