Consumer abuse

Order # ***

In december i did an online purchase of a car seat of 0kg to 25kg on sale for R1499. When the c****at was delivered it came as size 0kg to 18kg, and they advised me that i must accept that 18kg because when they advertised their sale online they used a wrong code. If i dont accept 18kg they will refund me. This is very unfair to me because its not my fault that they advertised the wrong thing, i feel my rights as a consumer are abused because in december a lot of stores had a sale and i could have purchased from other stores other than Babies r us should i have known they abuse their customer. I have suggested that they give me any brand of a c****at as long as its a size 0kg to 25kg but they dont care and say i either accept 18kg or refund. Where will i get value for money because now december sales are finished. I regret ever dealing with Babies r us.