Dude, seriously, how can she NOT believe you when you showed her the freakin' receipt!? Ask her if taking her to the store you purchased it at would suffice.
Just tell them how you got the money and show them the receipt, if they don't believe you then ask them why not and if they go "you must be a drug dealer" then just say that they're being bad parents by not trusting you with anything and its lowering your self esteem and that you worked hard to get the money. It should prove your case and you get to have something to use against your parents (which is very useful, trust me)
Go to the place you bought it from, use some of your drug money to purchase CCTV of you buying said product to prove to your parents that the means of procurement where 100% legitimate.