How can I ensure that my prepaid shipping label for the I-526 Approval Notice will remain valid throughout the 2-year processing period?

To the best of my experience, USCIS only mails by regular or priority mail USPS but does not use prepaid shipping labels provided with the Petition. If the Notice is lost in the mail (or you never receive it), you can request another copy.

We never ask our investor clients to prepay for any shipping label to send them their I-526 approval notice. Most lawyers will seek reimbursement for the actual delivery fees after they sent their clients' documents.

If you have an attorney, they will issue a duplicate approval. You should register for electronic notifications also. As for the shipping label: presumably, if your account is current, that should still work.

USCIS will mail the approval notice to the address on record (to the petitioner and to the attorney, if a proper G-28 was included). You can email the USCIS-IPO if the approval notice is lost in the mail. They usually will mail another one if you ask nicely.

They often ignore these anyway even when the approval comes quickly. Getting the approvals is usually not a problem though. If you move, be sure to file an AR-11 with USCIS.

It is not hard to obtain another copy of the I-526 approval notice. If all attempts to obtain a duplicate fail (which would be highly unusual), you can file I-824 for another original copy to be issued. Your attorney will (should) take care of all of this for you. It is not customary to send pre-paid shipping labels to USCIS; the agency has its own way of processing notices and I doubt they would use your label.