What should a Good Test Plan include?

— The best way of knowing what your test plan should include will require a good understanding of the objectives of the project deliverables.

Design Testing —

Check your website’s overall design.
Check for image quality across browsers/devices
Ensure that the colors of your site’s background and various design elements are correct.
Check font styles and colors.

The following are a few things to test:

Is it going to auto-scroll?
If yes, at what interval will the image be refreshed?
When the user hovers over it, is it still going to scroll to the next one? (When we have 2-3 banner images on the slider)
Can it be hovered on?
Can it be clicked on?
If yes, is it taking you to the right page and the right deal?
Is it loading along with the rest of the page or does it last in comparison to the other elements on the page?
Can the rest of the content be viewed?
Does it render the same way in different browsers and different screen resolutions?

Common tests are:

Search based on the Product name, brand name, or something more broadly, the category. For example Camera, Canon EOS 700D, electronics, etc.
Search Results have to be relevant
Different sort options have to be available- based on Brand, Price, Reviews/ratings, etc.
How many results to display per page?
For multi-page results, are there options to navigate to them
Also, search happens in many places. Please take the search drilling down into multiple levels into consideration when validating this functionality. For example: When I search on the home page, I might see something like this:

Check:

Image or images of the product
Price of the product
Product specifications
Reviews
Check out options
Delivery options
Shipping information
In-stock/Out of stock
Multiple color or variations options
Breadcrumb navigation for the categories (highlighted in Red below). If navigation such as that is displayed, make sure every element of it is functional.

Test the following:

Add items to the cart and continue shopping
If the user adds the same item to the cart while continuing to shop, the item count in the shopping cart should get incremented
All items and their totals should be displayed in the cart
Taxes as per location should be applied
A user can add more items to the cart- the total should reflect the same
Update the contents added to the cart- total should reflect that too
Remove items from the cart
Proceed to checkout
Calculate Shipping costs with different shipping options
Apply coupons
Don’t check out, close the site, and come back later. The site should retain the items in the cart

Check the following:

Check different payment options
If allowing check out as a Guest, simply finish the purchase and provide an option to register at the end
Returning customers – Login to check out
User sign up
If storing customer Credit card or any other financial information, perform security testing around this to make sure it is secure.(PCI compliance is a must)
If the user is signed up for a long time, make sure the session is timed out or not. Every site has a different threshold. For some, it is 10 minutes. For some, it might be different.
Emails/Text confirmation with the order number generated