Hi,
mojoPortal WebStore is not yet a full blown ecommerce solution, it is good for selling download products.
http://www.mojoportal.com/e-commercesolution.aspx
PayPal Standard is supported PayFlow is not.
PayPal and Google Checkout are the most economical payment systems available and I think it is wise to offer both, but there are some differences in what they allow. For example Google Checkout does not allow you to use it for donations unless you are a non-profit, so for example people can buy add on products for mojoPortal using google checkout but to Buy Joe a Beer only is allowed with PayPal. So we have a way to mark a product as a donation and disable google checkout if someone adds it to the cart.
Actual cost of using PayPal or Google Checkout is almost identical to each other.
I'm not sure of the costs for others such as Authorize.NET other than I'm sure they cost more and have monthly fees in addition to transaction fees, so one has to be doing a lot of business to justify the costs.
Hope it helps,
Joe