Klaviyo
Starting price: $20/mo on the Email plan for 500 profiles, monthly billing. Free plan: 250 active profiles, 500 email sends a month, and $5 of mobile messages.
What it does well for fashion. Klaviyo reads your product, order and browsing data and turns it into segments a merchandiser would recognize, such as shoppers who bought wide-leg trousers in a given size and paid full price. Drop flows, VIP early access and back-in-stock alerts all draw on the same segments, so a sold-out size triggers a message to the people who viewed it, not to the whole list.
Where it falls short. The cost grows with the profile count, and SMS is a separate plan with pricing on request. Rental and subscription billing is not its strength, so keep that logic in your billing system and pass the status into Klaviyo as a property. Small stores can find the setup heavy before they have much data to segment on.
Price and link. Plans start at $20 a month for 500 profiles and rise to $400 a month for 25,000 profiles on the Email plan. See the current Klaviyo pricing page.