Issues with providing tips to sellers

As I am familiar with great service, I typically try to tip the seller after the work is completed.  Upon completion of an order today, despite having a balance in my account, the tip function seems not to be working correctly, at least for me.  I have tried four times, refreshing the window, opening a new window.  Nothing works.  After you set the amount and click "tip," it brings you directly to the homepage.  Advice?  Am I doing something wrong?  Either I will come on here hours from now, and find that I truly did tip them four times and the website didn't update after doing do, or it isn't working correctly.  Thanks!
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