rear left door latch. door not get closed. open during drive

Ford fiesta 2009 left rear side door stay opened and get opened during drive.

i fixed it few times by playing with the door latch, but last time the door stay open and it was not possible to lock it. i asked ford garage for fix cost and they told me that i need to replace the latch and it will cost 300$ for parts, work & TAX (it is very expensive to fix cars in Israel).

the car is with very low KM due to low usage of the car and the car is in grate condition bet this door problem is pain in the ass...