You can easily enable shipping based upon a customers tag by using the code shown below.
Place this code in the storepickup-addons.liquid file in your theme.
{% assign disable_shipping = true %}
{% if customer %}
{% if customer.tags contains "Shipping Allowed" %}
{% assign disable_shipping = false %}
{% endif %}
{% endif %}In this example shipping would be disabled by default and not appear as an option on your shopping cart page. Shipping would only be visible if your customer was logged in and had the tag "Shipping Allowed".