"In the 40 years we have owned this store, nothing like this has ever happened," Toyworld Bunbury owners Harvey and Rick Baker said.
The brothers were working on what was just another busy day in December, when a man approached the counter.
"He just randomly came up to the counter and said I'd like to pay for the lay-bys," Harvey said.
"I was a bit confused at first, but he asked me to tally all the existing lay-bys up, then gave me his phone number and asked me to let him know what the total came to."
The total of the 50 or so lay-bys, which were mostly Christmas presents, came to around $13,500.
Harvey said when he called the man, who was insistent on remaining anonymous, he gave him his credit card number and paid all the lay-bys off.
"On behalf of both Toyworld Bunbury and all our customers - thank you. We're absolutely over the moon."
- Toyworld owner Harvey Baker
The pair decided not to tell the customers, instead opting to wait to reveal the good news when the customers came in to pick up their lay-bys.
Rick said everyone's faces lit up when they came in to collect their purchases.
"It was just a beautiful surprise for everyone, with a few people getting a bit teary," he said.
"Overall people were just gob smacked, because some of the outstanding amounts were pretty high, with one at almost $1,000 remaining."
Dalyellup-resident and Toyworld customer Steve Gare said when Rick told him his lay-by had been paid off, which had an outstanding amount of $80, he was both "grateful and overwhelmed".
"It just shows how kind people can be," Mr Gare told The Mail.
"Whoever the 'Secret Santa' is I hope he has had a fantastic Christmas and Happy New Year."
Eaton resident and Bunbury Mail journalist Jemillah Dawson was also one of the lucky customers who had their lay-by paid off.
She said she was in shock when she went to Toyworld to pick up her daughter's present, which had $90 remaining, and was told by a staff member, 'you don't owe anything because a 'Secret Santa' paid for it'.
"It was such a surprise that I kept asking if the staff member was being serious," Ms Dawson said.
"Then I thought I had no way of saying thank you to this 'Secret Santa' or for them to know how much it meant to have that happen.
"It was an amazing gesture which really brightened my day and boosted my Christmas spirit.
"Thank you again to the anonymous donor who helped their community at such an important time of year."
Both Rick and Harvey want the 'Secret Santa' to know how grateful they are not only for the joy it brought for their customers at Christmas - but for also supporting small business.
"In the 40 years we've been here, this is the first time this has happened. It's helped a local business where the money will stay in Bunbury.
"On behalf of both Toyworld Bunbury and all our customers - thank you. We're absolutely over the moon."
The good gesture follows another act of kindness from an anonymous good Samaritan who visited the Bunbury Forum's Big W in November and paid off over $700 worth of lay-bys.