mjaschen

Often it’s required to limit the cart to one item. This plugin achieves this by emptying the cart before 
a new item is added.
<h3>Requirements</h3>
<ul>
<li>PHP 5.6</li>
<li>WordPress >= 4.9.0</li>
<li>WooCommerce 3.x</li>
</ul>

WooCommerce – Allow Only One Item in Cart

This WordPress plugin ensures that only one item can be in the WooCommerce cart at any given time.

paxmanpwnz

Dippsy – the DIgital Products PayPal Store for You!
With it, you can easily sell your digital products, whether it be music, e-books, software, pictures, videos – anything that can be stored in bytes.
Payment gateway is the known widely used and hence verified Paypal.
Dippsy has many features. Some of them are:
<ul>
<li>Clean interface</li>
<li>Easy product adding/editing</li>
<li>Product preview</li>
<li>Multiple files per product</li>
<li>Many file formats support</li>
<li>Simple order review</li>
<li>Order status e-mailing</li>
<li>Cancel/completed message pages</li>
<li>Many customizable settings</li>
<li>Customizable store frontend look</li>
<li>Warning messages</li>
<li>Automatic PayPal IPN handling</li>
</ul>
For more information see <a href="http://dippsy.cz.cc" rel="nofollow ugc">official Dippsy website</a>.

Dippsy

An easy to use PayPal e-commerce solution with only downloadable goods in mind, hassle free installaton and many configurable settings.

WooCommerce – Allow Only One Item in Cart vs Dippsy