From a startup SaaS perspective, is it really that detrimental to have your customers sent to another site (Google Checkout or Amazon FPS, for example) and then

From a startup SaaS perspective, is it really that detrimental to have your customers sent to another site (Google Checkout or Amazon FPS, for example) and then brought back after the conclusion of the sale? Do enough customers walk away if the payment system is 3rd party visible as opposed to 3rd party behind the scenes? For any small startup trying to minimize costs, it seems like eliminating those monthly minimums and setup costs might outweigh the benefits of a more seamless customer experience. Can those with firsthand experience chime in?