Are you being serious?
You, youself, wrote in the OP:
"The Digikey order clearly stated "Incoterms: DDP (Duty and customs paid by Digi-Key,
VAT due at time of delivery)""
So the discrepancy
cannot be about anything else than the 3.62EUR. I accept if you are after this 3.62EUR "by principle", until you get a proper explanation, or refund for that. I have principles myself, too, but I strongly recommend to think critically whenever you find yourself twisting the truth for your own gain.
But what you just wrote - total and utter bullshit.
Yes, small packets can get past customs "by luck" without you "needing" to pay VAT, but this luck, IME, does not happen with UPS. Also note while you talk about VAT being "waved", it's possible that you are actually committing a possibly criminal offence (albeit meaninglessly small) by not paying the VAT, unprompted; depends on your local jurisdiction. Why are you counting on this and require Digikey to be responsible for the fact you weren't lucky to be able to sneak past paying the required VAT, which you were informed about beforehand, and which you clearly acknowledged from the beginning? You clearly agreed to pay 10.71 EUR at the time of delivery, and now you are wasting Digikey's time arguing about
that 
.
By all means go on and fight for the unexpected 3.62EUR if you feel it's necessary for principles.
But it's clear at this point requesting refund for VAT is not about any sound principle, just you lying to yourself, or being a dickhead. Digikey is being responsible for not "refunding" you anything, because that would be then subvented in Digikey prices for me to pay.