Conference registration fees by sketchy means?
(Not an answer but a long comment)

Sending credit card numbers via e-mail is a violation of the security standards of the credit card industry. See, for instance: 


Does sending credit card info over email violate PCI-DSS? 
Is storing CVV complaint with PCI standards?.


If you report the conference to your credit card issuer, they will risk a fine. The conference will then have to switch to another means of communicating credit card numbers that complies with these directives, such as an electronic payment form, fax or snail mail (sigh).

Note that these are indeed arguably more secure than e-mail, because they are much less vulnerable to remote hacking.