There’s a certain class of product for which you’re much better off to buy online. Specifically, you’re looking for things that are commodities (but not dollar-store items), durable, small, and lightweight. Here are some things where you’ll find a much better deal on e-Bay, etc.:

• Brother-compatible labeling Tape (aka TZ-Tape) as low as $5/cartridge
• Ink Cartridges
• Button cell batteries (leading brand CR2032’s for less than $1 ea.)
• AAA Duracell batteries, packs of up to 100
• Car lighter USB adapters (especially the high power 2.4 amp ones)
• All electronic parts, obviously

For AA batteries, apparently there’s evidence that Costco’s Kirkland brand AA batteries are actually rebranded Duracell batteries, so I can’t find a better price than them online.

Since practically all electronics parts are made in China, you can get them on AliExpress.com for extremely low prices and you can usually get free shipping. I regularly purchase Arduino compatible or ESP8266 boards, prototyping supplies, power supplies and sensors mail order from AliExpress with free shipping, all for less than $10, sometimes as low as $1.80 with free shipping, and the only down-side is that you have to wait a month or two.

If you need it faster, and it’s a more popular item, it’s likely there’s a small importer that has some for sale on e-Bay in your own country, for a slight markup. This is true for batteries, chargers, ink and tape cartridges and usually for popular microcontroller boards.