Walmart family mobile Miss leading prepaid calling servicet, Internet

Was told by walmart cs rep phone service was prepaid no contract can discontinue anytime. Stop using phone service march paid last bill march.  Received last bill may 5th dated april 29th 179.57 for 2 months. Called 1-877-760-8760 to dispute. was told not a prepaid service by rep. plan is a post dated plan.  advised rep was not explained plan was post dated and nothing in writing when purchasing plan stated post dated. Feel walmart rep mislead or was misinformed with product knowledge. Tried to file complaint unable to do so with 800# provided above.  This is my only option...