if a contract is being terminated and I said in the contract that i will send them the termination papers registered mail and

Technically it doesn't comply if it doesn't do exactly what the contract says. Whether or not the termination is still valid, however, is a larger question involving the language of the contract and your state's substantive law.

Disclaimer: This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.

If your contract specified that termination could only be done by registered mail, then Fed-Ex, while similarly provable, would not suffice.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.