Usability of your website might be a supporter of your sale but if you do not design it properly, it will become a threaten. This article for Magento site, I will focus on how Magento extensions protect your website from usability issues:

1. Load time

Visitors, nowadays, are accustomed to fast web pages and I bet that they have no reason to stay on a slow website. Visitors are not such patient. From the beginning, I recommend you to choose a server that will be able to accommodate your website’s traffic and data needs. Stay away from the unnecessary animations and large images, the speed will be improved significantly.

2. Navigation

• Make it easy to find: Never let customers get lost in the matrix of navigation, it will like you do not encourage them to make impulse buys and will likely discourage sales altogether. So, spend the time to categorize your merchandise and implement an efficient site-wide search function.

• Consistent navigation: There is no certain standard for how navigation must be but it is recommended that links to next page or previous pages should be in the same place on all pages of your site and should be easy to recognize. Only add related products to that page and checkout process should be displayed in easy-to-see place, generally in the right-hand side.

• Mega menu: it is the savior for your website, especially, it is the must-have item for medium and huge websites. The compulsory elements on menu would be Home, Contact us, Shopping cart, you can add anything more as your requirements.

Benefits of horizontal mega menu:

• Majority of languages are read from left to right so a horizontal menu bar will be more efficient for visitors to read.

• Horizontal menu converses more vertical space

• Top navigations are easier to find because they are usually accompanied by the header and logo, both of which are visually dominant objects on the webpage.

Benefits of vertical mega menu:

• Visitors tend to click on the next link to view a different page.

• It will work really well for single page layout

• While a vertical menu takes up the left side of your webpage, you gain vertical space for your content.

Refer:Magento Layered Navigation extension and Magento Mega menu extension, Magento Calendar Printing Website ThemeMagento frequently bought together extension

3. Checkout Process

• Make the cart clear: What does clear mean? Cart icon should be highlighted and easy-to-be-seen enough. You can add shipping cost to inform customers but avoid adding some surprise cost related to processing, handling or tax, they do not belong here, they should be in checkout process, don’t annoy customers.
• Keep it simple: this is an obvious point but I see many websites refuse to follow. Generally, a checkout process will include 5 or 6 separate pages and each page might contain the risk of customer leaving without completing sale. Never ask too much information if it is not really necessary. Checkbox is recommended item, it will help to fill automatically in shipping details when they match the same as the billing data. However, the most important is reducing multipage checkout to One Step checkout to ensure that everything in just one step, no more page to load, no more time to wait.
  Don’t require registration: Registration should never be compulsory when customers make a purchase, you only should encourage them to register by giving them some benefits.
• Refer: Magento shipping cost calculator extension, Magento One step checkout extension