If you add no weight to the shrimp then they will be Free Shipping when 0 weight is defined as Free Shipping in the Shipping/Packaging and you need to install the Free Shipping freeshipper shipping module ...
If you want to change the language for this, you can customize the language file by copying:
/includes/languages/english/modules/shipping/freeshipper.php
to your template and override directory:
/includes/languages/english/modules/shipping/your_template_dir/freeshipper.php
This however is used for ONLY showing Free Shipping and no other shipping module ...
Another way to do it is to set the weight to 0 but set the 0 weight does NOT mean Free Shipping and install clone the Flat Rate flat shipping module to something like flatfree ...
This will show with other shipping modules ...
Now customize the code for the new flatfree shipping module to only display for that one Product or Category ...
Code is in:
/includes/modules/shipping/flatfree.php
Language is in:
/includes/languages/english/modules/shipping/flatfree.php
Set it to display when weight is 0 ... or when just Products you want are in the cart or products from a given category (master_categories_id) is helpful here to determin that ...
Somewhere in the last few weeks I just wrote up an example of how to control this and have it show when a products_id or categories_id is in the shopping cart ... but I forget where I put that ... :unsure:
Now this module will show only when these products are the only thing in the cart ...
Make another clone of Flat Rate flat for the flatovernight ...
When these products are in the cart with other things they will not add weight to the cart so they can ship for no additional charge ...