Install once, then uninstall and then it gets added to your account permanently.
Tap on install and then cancel the download.
Quoting from this website: Learn about refunds on Google Play - Google Play Help
You may get an automatic refund if you uninstall a paid app shortly after first buying it. If you want to reinstall the app, you'll have to buy it again and you may not be eligible for a refund on that purchase.
I had installed a paid icon pack from Play Store that was on sale few months back. I uninstalled it after trying it out, thinking that I'll install it on my second phone. Few days later when I tried installing it on my second phone, I was asked to pay for it. Checked it on the first phone, where I had originally downloaded the app, and on that too, I was supposed to pay to install it. Googled up and got to know that if the app is uninstalled shortly after installing, it is removed from your account.
Don't know if others have faced this issue, but I now make sure to keep the app for a week at least whether I need it or not.