[NickyDee]

I'm not sure what level your coding is at but on method of achieving this might be to stick some javascript into the registration template.

e.g. onClick="runFunction()" would go in the user profile field form. Then in the function something like;

runFunction() {
var profilefield = '<vb:raw userprofilefieldX>';
if (profilefield == '3 months') { window.open('http://www.site.com/billing.php?duration=3','mywindow'); } else if
(profilefield == '6 months') { window.open('http://www.site.com/billing.php?duration=6','mywindow'); } else if
(profilefield == '1 year') { window.open('http://www.site.com/billing.php?duration=1,'mywindow'); } else if
(profilefield == 'LT') { window.open('http://www.site.com/billing.php?duration=0,'mywindow'); }
}

I haven't tested this and it will probably need modification to work, but its just to give you a general idea how you might achieve this.