I'm not quite sure how much i actually condone or recommend doing these but here's some ideas anyway
Kinda scummy but if her birthday or some other special event is round the corner you could say it's for that and that you had to buy it online. Or maybe you could say it's for a friends birthday and that you don't want her to ruin the box. You would of course have to substitute it with a real present and put it in the same box.
You usually get a rough estimate of when a parcel should arrive (i.e 2-3 days). If you do buy time it so it'll come on the weekend. If packages usually arrive a little late than estimated then buy the day before. Or alternatively some places let you choose a specific day for something to be delivered, i believe they just keep it in storage or something until the day so that it'll definitely come at that time. if you did this you'd have to give it enough time to actually arrive for the selected day, I'd recommend buying at the start of the week and have it delivered on Saturday, having Sunday just in case it's late.
Maybe you could ask if you could try buying something online, obv not a sex toy but just something. Say you want to learn or you can do it yourself or something, if she asks to see what it was or wants to be there or she checks the bill it'll all be something "normal". This way you can next buy the toy and act like it's just like before when you bought the smaller normal thing. You just gotta make sure you get it right the first time and that you do know what you're doing.
Here's some websites with discreet shipping and details
I've bought from Lovehoney (actually in a pretty similar situation) and it was great, super secure and secretive.