Because if you park around the corner the cops can't see where you walk to with arm fulls of stuff? I think that if you are trying to be all sly about things, and start acting all shady, if by chance the place is being watched, you'd be the first person they'd follow. Not the guy who is acting like he is doing nothing wrong. Which he isn't because buying garden supplies is not illegal.
Don't trip about your credit card usage. Shopping there is not probable cause to get a warrant, search your house, or what ever. Calm down, the cops have bigger things to worry about than you going and spending $100 at the hydro shop. Shit, the cops don't know how much a bottle of nutes cost, you could walk out with $500 worth of stuff in one bag without a problem. I watched a guy drive off in his truck with 22 bags of ocean forest soil and a bunch of giant tubs of nutes. No bed cover or anything. Just cruising around with all that stuff.
If you are not selling weed, your neighbors don't hate you, and you can't smell it growing outside your house, why would they have any reason to believe you are growing weed? There is nothing for them to go on and thats just what a judge would say when they went to get a warrant.