Excellent plugin. Please keep up the good work.
I came across a situation, maybe unique to me, one of my client said he is not comfortable using a invoicing system and paying using it since the system is not on a secure server (my website does not have SSL) and therefore he would be comfortable using a 'known' invoicing and payment system.
The argument I gave the customer was that all the transaction happens not on my website but on the payment gateway like Paypal therefore at no point of time he is at risk as he is going to disclose any confidential information on the payment gateway site which is fully secure. (I hope this is correct ;-) ).
Please help me with more points I can put forward in case a similar situation arises in the future.