It all depends on how the plugin author chooses to do business. If they let Trimble handle the sales then currently Trimble doesn't work with paypal, just credit cards.
Other plugin authors who have their plugins on Extension Warehowse handle their own payment strategy. Often they do use paypal.
I have 2 plugins that are available from Sketchucation's Plugin Store and also from Trimble's Extension Warehouse. My strategy is I give you a free trial period and then if you want to purchase a license you go to my website and use paypal.
Other important things are what kind of a license - subscription, perpetual, major version upgrades etc. How many computers can you install the plugin on? etc. etc.
Plugin author's may allow you to send a money order etc. Send them a PM (personal message) and ask.