The letter and voucher were a "slap in the face", said the woman, who does not have a car.
She went to Wellington's Wakefield Specialist Centre on March 20 for a colposcopy after an abnormal smear test but the acetic acid used in the procedure should have been diluted to 5 per cent but was instead 100 per cent concentration. It caused severe internal burns.
Newton Mall Pharmacy labelled the acid and is investigating what went wrong.
Three days after the incident the woman received a letter from the pharmacy, which apologised for the bungle.
The letter, signed by directors Mark Newton and John Phipps, said the company was investigating the cause of the incorrect supply, as well as "reviewing our dispensing procedures and checking systems relating to the supply of items to the clinic to prevent future errors".
"Please accept the enclosed voucher to cover your travel costs relating to your readmission to the clinic," it added.
The traumatised woman said the letter was of little comfort.
She told the Herald on Sunday she endured "horrific pain" while the undiluted solution was applied to her. But she thought the pain was part of the procedure.
The pain intensified over the next 24 hours, when she was at home.
She realised something had gone terribly wrong after being contacted by the doctor who carried out the procedure and asked to return to the clinic for a check-up.
"They explained that they had done the procedure on another woman but she was in heaps of pain, so they sent the solution for testing and it had come back that it was 100 per cent concentration not 5 per cent," she said.
For the next three months the 21-year-old had to visit the hospital for treatment to the severe burn, spending an estimated $1500-$2000 on medical bills.
The woman said she had also endured immense stress.
"I wasn't allowed to walk anywhere. I couldn't sit down because it hurt so badly."
Her medical files revealed the severity of the burns, which affected 80 per cent of her vagina.
"I hadn't realised that I probably would have needed surgery if I hadn't healed properly," she said.
She said the surgery would have involved a reconstruction and skin grafts and could have affected her ability to have children.
The files also confirmed another woman had been affected by the error after the undiluted acetic acid was also used on her too.
Dr Fali Langdana, who performed the colposcopy at his clinic, said Wakefield Specialist Centre's chief operating officer had met with pharmacy bosses after the incident.
At the meeting, he said the pharmacy's directors agreed to carry out a full investigation and its ordering, dispensing and checking provisions.
Langdana's practice has since stopped ordering supplies from Newtown Mall Pharmacy.
The woman is now seeking compensation from Newtown Mall Pharmacy for her medical bills.
Her family has also lodged a complaint with the Health and Disabilities Commissioner (HDC).
The HDC confirmed on Friday a complaint had been laid but would not comment further.
Neither Newton nor Phipps would comment when approached by the Herald on Sunday.
