There are several ways to send the message to seller.
Step 1 : In item listing page you can find the “Ask a Question” link. By clicking this button you can contact the seller.
Step 2 : From the shop profile you can also contact the seller by clicking “contact shop owner”.
Step 3 : After completing order you can also contact the seller from the purchase & review page, by clicking “Contact the Shop” button.
Step 4 : In cart page also you can send the message to the seller. Once you logged in to your account, you can see the link “contact shop owner”, then click that link to contact seller.