Use non-technical staff for the testing, since they don't make implicit assumptions about how it works. Assign clear roles to the people handling different parts of the project. They have enjoyed great savings from on-line entry of registrations for conferences, as opposed to entering data from paper forms. Registration fees are sent by credit card with the registrations. Test some more.
Some browsers lock up at the start of the encryption process.
Multiple-clicking on the submit button can cause multiple charges to the credit card. That emphasizes the need for immediate response to let the user know that something has in fact happened. Keep central control of the entire process. In the future they aren't going to offer the PDF option, which allows users to download a paper form and fill it out to send in. If they resist using a credit card, have them call you and then send them a fax.