Hi Rapidturtle
Better late than never, right 
IOSS is simplification for goods shipped from outside of EU to EU end-customers with intrinsic value below 150€. IOSS is applicable for both EU and non-EU merchants.
DDP does not require IOSS. IOSS provides simplified customs procedure and removes the need for carrier to pay taxes to customs. Also, it is a simplification on the fiscal side, meaning single registration, filing and payment of VAT.
DDP service means that the carrier pays the taxes and duties if applicable to destination country customs (IOSS can be cleared in any EU customs). There are no value thresholds. Customs clearance is the full and therefore slow and expensive. If you have the IOSS number, ask for IOSS services from your carrier, they are significantly cheaper than DDP.
Feel free o reach out with more questions to us at EAS Project.
Shopify app link: https://apps.shopify.com/eas-eu-compliance