A receipt alone won't do it. You also have to prove you are now the rightful owner of the deceased person's iPhone.
Activation Lock for iPhone, iPad, and iPod touch - Apple Support - This is a security feature, in part to deter re-selling of stolen devices. --> http://support.apple.com/HT201365
Read this link to find out how to "Turn off Activation Lock" but note an activation lock can only be removed using the log-in information for the account that applied the lock. --> https://support.apple.com/HT201441
The official owner of the deceased person's things as can be proven by documentation needs to try one of these things:
Read this link to discover "How to request access to a deceased family member's Apple accounts" [and devices] - https://support.apple.com/HT208510
If you are only trying to remove an activation lock the new owner can try taking it to an Apple Store along with proof of identity, the original sales receipt for the device with the deceased's person's name on it, and documentation (will) indicating ownership has passed onto the person submitting the item. There is no official policy about this so it is up to the store manager if they will do it for you.