My 2007 return was postmarked 4/18/11

Actual IRS reference that 4/18/11 is legitimately the last day (most are third party):
IRS Has $1.1 Billion for People Who Have Not Filed a 2007 Income Tax Return

I was told by an IRS representative on the phone before I filed 4/18/11 was the last date. I was also told I was good during a conversation where I called to confirm receipt.

I am currently trying to find my certified mail receipt but having a bit of trouble. I was careless with it once I was told by an IRS representative on the phone it was received and I was good!

I know in this case I feel strongly I am in the right. I just need help/insight on how to BEST proceed. Thank you in advance.

“I received a letter disallowing my 2007 refund for being filed untimely - more than 3 years - but it was postmarked 4/18/2011. What is the BEST and FASTEST way for me to pursue an appeal? “--->When a refund has expired, that refund money disappears forever. In IRS terminology, an expired refund is considered an "excess collection". That refund money cannot be sent to the taxpayer as a check. Nor can the refund money be applied as a payment towards another tax year for which a person might still owe the government. Nor can be refund be applied to another year as an estimated payment.
“I am currently trying to find my certified mail receipt but having a bit of trouble. I was careless with it once I was told by an IRS representative on the phone it was received and I was good!I know in this case I feel strongly I am in the right. I just need help/insight on how to BEST proceed.”--->I guess you need to contact the post office that issued the receipt. Hopefully, they keep a log of registered mail deliveries and the record of obtaining the addressee's signature will suffice.