I understand the issue here. Unfortunately, the policy decision chosen by legislators is to use a system that gives a specific amount of time for a piece of mail to be delivered, and add that period of time to the date of mailing to give a "deemed" date of service.
So if two documents are issued by the party on the same day, one by personal service, and one by US Mail, the document served by personal service is "deemed" served on that day, and the responding party must calculate their response accordingly, while the document served by mail is not deemed served until 5 days later (giving the recipient an additional 5 days - this is true whether it takes 2 days for the parcel to be delivered, or 5).
I do understand the frustration (and I sympathize with it, I am a civil litigator and I have to deal with these codes daily, dates of service are not uniform, and some codes give additional time for mailing, others do not, it is incredibly frustrating), but unfortunately, this is the system that we have to work with. Until legislature changes the system, as litigants we must work within it or we will compromise our claim.