Exceed Limit ? - Premium User and not over 50MB OST

I am trying to convert OST to TXT(or ZIP, pst… ) but failed and failed again and again. and it appears exceed limits because of file size?
it is 30MB OST file. and I just purchased premium user for this and I have no time for this…
please let me know how can I proceed this or I need to refund it… if it is not possible.