Same here, I refill jugs at walmart, but they charge 33 cents at mine grrrr.
Make sure you check the PH of your topoff water. The water at walmart has a PH of 7. This may lower your tank's PH over time and require additives.
ps- the water from the RO machine at walmart is better than a home RO unit. It does carbon filtration, UV sterilization, reverse osmosis, ozone, and one more I think. Gets expensive though, about $3 a week.