I would really advise against providing your credentials to a third party developer/company. The way Amazon handles this is that they need to provide you with their developer id so that you may authorize them to get access to your account. This way you can revoke their privileges at any time.

FYI This link says your Merchant Token is also known as your “seller identifier”
https://sellercentral.amazon.com/gp/help/48321

Your Merchant Token (aka Seller ID) is found here:
https://sellercentral.amazon.com/sw/AccountInfo/MerchantToken/step/MerchantToken

Your Marketplace ID is here (choose one depending on country):
http://docs.developer.amazonservices.com/en_US/dev_guide/DG_Endpoints.html

You need to get the MWS Developer ID of ShipWorks, then authorize the ShipWorks Developer ID to access your seller account. You add their Developer ID to your seller account here::
https://sellercentral.amazon.com/apps/manage

After their Developer ID has been authorized to access your seller account, the “MWS Auth Token” is created and is available on that same screen. Press the “View” button to seen that value.

David Nelson
Dynamic Enterprise Technologies Inc
Seattle Washington USA

If you google
shipworks adding amazon store

you will find a nice support document to setup your amazon account integration. You do not need to get developer credentials, you will go through the manage app screen and use the ‘Authorize New Developer’ button to change their developer id into an auth token and sellerid to put back into their forms.

EDIT: another new updated screen messing with everyones docs

Autonomoose is right, the help file is pretty straight forward. If you hit a roadblock, call Shipworks.