Yes! You can purchase a bucket of credits without having to upgrade your plan!
First, go to your "Billing" page. (Click your name in the top right corner, then select "Billing")
Next, Select "Buy More Credits"
Third, choose the size and price of the bucket of credits you wish to purchase by clicking "Add Now".
Next, add your credit card information in the fields below OR choose a saved credit card to charge (if applicable).
And finally, click "Charge" to immediately charge your credit card and purchase your bucket of credits.