how to collect VAT during checkout for EU customers?

[blazeb2b]

Additionally I would also want to do some sort of validation to make sure entered VAT ID is correct.

[Agreeableagony]

I assume this is regarding a store that is outside the EU and shipping to the EU. 

Assuming that is correct, there are a few options.

1. If you are signed up for the "Advanced"  Plan shopify gives you the option to calculate and collect EU Vat and Duty at checkout by default.

2. If you are on a lower plan, there are apps that offer this as part of Calculated Shipping  - such as www.easyship.com   - They will calculate a landed DDP cost and provide that to shopify on the cart page as the shipping price  (it makes shipping look wildly high, but as long as you are clear as to why....its fine).
 
With option 2 you will need to activate Carrier Calculated Rates which is a small monthly add-on with shopify, just talk to support: https://help.shopify.com/en/manual/fulfillment/setup/shipping-rates/third-party-carrier-calculated-s... 

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As far as VAT ID   with option 1 above you can either use your VAT ID (registered with an EU country)  OR leave it blank and use a shipping carrier that bills the tax and duty back to you.

With option 2 you are just giving the money to carriers to handle it so you are not providing a vat id anywhere.