Avoiding manual floor maintenance is a lovely thing, but a good robot vacuum will cost you. Our current favorite pick for a mop and vac combo, iRobot’s Roomba j7+ usually sells for $1,099 but Wellbots will knock $300 off the list price when you use the code 300ENGADGET at checkout. That beats a $200 discount we saw earlier this year and represents an all-time low for a gadget that…