Hidden fields are not more secure than parameters in the URL.
('show source' in your browser and everything is revealed...)
But they look more 'professionnal' to me, even if I admit it's purely subjective and aesthetic...

To me one way to secure a parameter would be to compute a hash (MD5,SHA1)
of the parameter, the id session, and a private value (the customer ref in your base, for example)
This way the link can't be cut and pasted later (beccause of the ID session)
And link can't be faked given an id-session
(as the private value is needed to compute the hash...)

UPDATE :
Just my 2 cents idea, Cookies would solve a lot of your troubles here,
a lot of professionnal sites use them.
And you may be surprised to see how well the customers accept this constraint
('Yes sir! your browser need to be cookie enabled to use our site.'
is a sentence that I often say and it's always ok for my customers...)
"Only Bad Coders Badly Code In Perl" (OBC2IP)

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