Hi Gitesh,

1) This has been discussed some time ago in our Forums:
devnet.kentico.com/Forums.aspx?forumid=36&threadid=9954
That Pavel's suggestion is generally still valid.

There are some connectors for taxes (Avalara) available at our Marketplace, but not directly for any exchange rate service.

2) Is this dropdown a custom one or our default control?

To switch the currency of the current shopping cart, which will have effect on a prices in product listings too, you could use following code:

if (ECommerceContext.CurrentShoppingCart == null)
{
    ShoppingCartInf cart = ShoppingCartInfoProvider.CreateShoppingCartInfo(CMSContext.CurrentSiteID);
    cart.ShoppingCartCurrencyID = <ID of a currency you want to use>;
    ECommerceContext.CurrentShoppingCart = cart;
}
else
{
    ECommerceContext.CurrentShoppingCart.ShoppingCartCurrencyID = <ID of a currency you want to use>;
}

Any further details would help.
Please let me know if I can be of any further assistance.

Regards,
Zdenek