Local Funding Sources with Checkout - help getting set up

[scottpayne]

I'm trying to evaluate some of the new Checkout.js features and am currently looking at the Smart Buttons set up. I'd like to be able to submit a payment using local funding sources, the one I'm trying to work with right now is iDEAL in the Netherlands.

I've managed to get the buttons to show up by reading through the source code of the JavaScript and adding the following config:

    paypal.Button.render({
        // smart buttons
        locale: "en_NL",
        style: {
          layout: 'vertical',  // horizontal | vertical
          size:   'medium',    // medium | large | responsive
          shape:  'rect',      // pill | rect
          color:  'gold'       // gold | blue | silver | white | black
        },
        env: 'sandbox', // Or 'production'
        commit: true,
        funding: {
          allowed: [
            paypal.FUNDING.VENMO,
            paypal.FUNDING.CREDIT,
            paypal.FUNDING.IDEAL,
            paypal.FUNDING.BANCONTACT,
            paypal.FUNDING.GIROPAY,
            paypal.FUNDING.SOFORT,
            paypal.FUNDING.EPS,
            paypal.FUNDING.MYBANK,
            paypal.FUNDING.P24,
            paypal.FUNDING.ZIMPLER,
          ]
        },
        // ...

When I try to submit the payment I get the following message:

I'm using the server side configuration as outlined in the Server Integration Checkout docs. I've tried passing through the shipping address as the Netherlands as well as setting the locale, but I keep getting this error.

What is involved in allowing buyers to pay via iDEAL? Is there some configuration I need to pass? Is there some configuration on our sandbox merchant account I need to set up? Or is it something specific to the buyer - e.g. do I need to set up a sandbox buyer specifically based in the Netherlands? Is iDEAL just not available for sandbox accounts?

Thanks in advance.