Outsourcing online payments and Seaside apps

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> Hi again,
>
>     anything particular to Seaside one should have in mind to evaluate
> online payment providers? I was informed that the online payment provider
> makes it's approved (or nor) payment answer to an url with a post. As
> Seaside urls are changing for each page what safekeepings at that time
> should one take for Seaside or the provider should guarantee?
>
>     thanks,