Part of the $45,000 bill included a cross-country trip on an Alberta government plane to Ottawa, so that Redford could meet Prime Minister Stephen Harper's delegation heading to South Africa.
Redford's office said earlier this week the premier had to take the government plane because there were no commercial flights available.
But the Opposition Wildrose party noted Thursday that Air Canada had a flight landing in Ottawa about two hours before Redford had to be on the tarmac with Harper.
An Air Canada spokeswoman confirmed there were open seats on that flight, which involved a trip from Calgary to Winnipeg and then from Winnipeg to Ottawa.
"While we cannot disclose specific numbers, both flights on Dec. 8 departed with open seats," Angela Mah confirmed in an email.
WestJet also had a direct flight from Calgary to Ottawa that day, but a spokesman declined to disclose seat availability.
Redford's spokeswoman, Neala Barton, said in an email that the premier's office was given two flight options from bureaucrats in the International and Intergovernmental Relations Department, but neither could guarantee Redford would arrive in Ottawa on time.
"As a result, the decision was made to take the government plane," Barton wrote.
She said the Air Canada flight was not one of the options presented, but she declined to discuss why.
Redford is under fire for racking up a $45,000 bill to take herself and an aide to South Africa, even though flights there and back, as well as accommodations, were taken care of by the federal government.
In contrast, Nova Scotia Premier Stephen McNeil took the same trip to Mandela's funeral for less than $1,000.
Wildrose finance critic Rob Anderson said Redford doesn't understand how the issue has affected taxpayers on a visceral level.
"It's not the billion-dollar boondoggles that seem to have the most effect on people. It's the ones that they can understand, that they can relate to," he said.
He said $45,000 is as much or more than many Albertans make in a year.
Redford said Wednesday she, too, was upset over the cost of the trip, but didn't book the ticket — her staff did. She said if she had known the cost would be that much, she wouldn't have gone.
Redford worked with Mandela before she entered provincial politics and went to the funeral at Harper's request as his guest.
Her office has said the rest of the trip costs break down this way:
Redford's assistant, Brad Stables, was not allowed on Harper's plane so flew to South Africa at a cost of $10,000.
Both Redford and Stables, rather than take Harper's free flight home, left hours earlier on a commercial flight, at $10,000 each, to be back in time for Redford to attend the swearing-in of her new cabinet.
The pricey trip has sparked sharp criticism of Redford in Alberta. One newspaper cartoon lampooned her as ersatz royalty, a crown on her head as she strolls to her limo with a taxpayer trailing behind and weighed down by bags and suitcases. A columnist referred to her as "Princess Alison."
The flight flap comes at a time when her Progressive Conservative government has struck wage austerity contracts with teachers and doctors and has passed legislation to impose similar measures on its largest public sector union, the Alberta Union of Provincial Employees.
Her government has run deficit budgets and is on track to accrue $17 billion in debt by 2016 to pay for capital projects. Redford delivered a belt-tightening budget last year and cut funding for advanced education.
Finance Minister Doug Horner has promised a similar budget on March 6.
Guy Smith, head of the provincial employees union, said the $45,000 affair just widens the gulf between Redford and ordinary working Albertans.
He also criticized Redford for blaming her staff.
"As a leader of a large organization the buck stops with her," said Smith.
"It's never appropriate to blame someone else for something you should be aware of."
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