Instead, she gave it to someone who she knew could use it even more than she could.
Nelson owns Urban Sparrow jewelry, where she pays different venders to sell hand-made pieces. She says Shields, who is one of her employees, is different than the others.
"She is the only person who never takes a paycheck. Ever. She always tells us to donate it to a different charity every single month," said Nelson.
When a local elementary school teacher died, Shields raised money to buy a reading bench at her school.
She's also sold pieces to raise funds for families combating substance abuse, all the while sending support packages to her Marine Corps son. Nelson says Shields clearly lives generously and always pays it forward.
But when Shields stopped by Nelson's house to be surprised with $500, she decided she would turn around and use her money to pay it forward to someone else instead.
"I feel like giving it away, like, I don't think I should have it. It's really weird for me," said Shields.
She couldn't stop thinking about Austin Munoz, a Moorpark High School football player battling brain cancer. After successful chemo last year, Munoz's tumor began growing again and he just found out he needs another surgery.
His single mom, Julie Miller, also needs to take off time from work to be by his side. So Shields wanted to give them her money and invited ABC7 to surprise them at lunch.
"Somebody paid it forward to me. And I decided I wanted to pay it forward to you," Shields told Munoz and Miller.
"With all the news we just had, knowing I'm not going to be able to work for a few weeks, Susan is just our little angel, she just pops out of the woodwork!" said Miller.
To Munoz, Shield's gesture means a lot.
"It means I don't have to see my mom worrying about finances like she (Miller) said," he said.
Shields explained why paying it forward seems to be a way of life for her.
"Someday I'm sure there is going to be a time when I need something," she said. "You don't do it expecting anything in return. But it makes you feel good, you give what you get."
Since Shields gave Miller and Munoz the $500, Munoz has suffered several setbacks in his recovery. On Wednesday, he had to be rushed by helicopter back into the hospital.
To send messages of support to Munoz or to donate to his family, you can visit the Austin's Fund website.