Is this an issue there's no answer for yet?

Thanks for the reply.  Sorry - I should have been clearer. 

A 3 party transaction (in the jargon I'm familiar with) is one where the shopper comes along, fills their cart, clicks check out, and then is redirected to a payment gateway to enter their credit card details (using the gateways secure site), and then they're returned to the shopping site.

A 2 party transaction has the shopper fill their cart, go to the check page within the shop, enter their credit card details, click submit, and then the shop uses a webservice or some other technique to talk to a payment gateway behind the scenes, and the shop then shows the result to the user.

As I understand the two gateways in place at the moment - The paypal is a 3-party system - i.e. the shopper is redirected to paypal to complete the transaction.  So there's no need for the shop to be operating under https in this scenario.

With authorize.net, I think (but may be wrong) that the shopper enters their credit card details directly into the shops checkout page.  If the shopper enters their credit card details into the shops checkout page, then surely the checkout page needs to be operating under https?  If this is the case, how do you flip the checkout page over to https mode?

Does this make more sense?