You can call or email them to have your email address changed. They usually reply to emails after ~5 working days. From reading other posts on the forum, it sounded like it's hard to call them. One poster said he/she succeeded after calling them 121 (!) times.
They ask that you include an application receipt when you send the fingerprint. The receipt is sent as a PDF attachment in an email after your application is paid for.
In my email from May 2019, the address was:
To complete the application, please mail your original hardcopy fingerprint impressions recorded by an authorised fingerprint recorder together with a copy of the payment receipt to the COC Office at:
Block D, #02-07/08
Police Cantonment Complex
391 New Bridge Road,
Singapore 088762
Yes, that fingerprint form is correct.
P.S. If you really can't find that PDF receipt, I'd say attach whatever you can to show that you applied AND paid. Definitely include your application number. You should have had opportunities to print out your online application after you submitted and before you exited the website, and to print out your payment receipt after online credit card payment. If you did print those or took screenshots, include those. Those basically show the same thing -- that you applied and paid, and your application number.