No it’s not like Avon, it’s like being the cool girl at school that everyone copies. Except monetized
As noted above the best model is the one where your followers get a certain code or click on a link in your instagram and then you get a cut of every sale. Small companies can’t necessarily afford this but it’s the norm in the “real” fashion industry and the instagram models who work hard at it (taking good photos, styling things well, being supercool etc) make bank.
If a company comes to you with a business model other than this, it still might be worth it for you if your instagram isn’t that popular or you are getting a decent discount that will “pay” you more than the expected commissions. You can research online to see what % of your followers you can expect to make a purchase at each price point etc but keep in mind most data is for the fashion industry. I believe some workwear blogs function this way too and that’s more similar as people “have” to purchase corporate wear.
Anyway you’re not going to get rich off the horse person apparel industry but don’t sell yourself short if you are like the top bigeq rider in the nation!