You list a jacket for $80 on eBay. It sells. You receive about $64. Where did $16 go? Final value fee, payment processing, maybe promoted-listing spend — each one small, together a fifth of your revenue. Most sellers learn this math backwards, after months of "sales are great but I'm broke." These calculators run the numbers forward, before you buy inventory.
Amazon FBA: the fee stack nobody warns you about
Amazon's fee structure has layers. There's the referral fee (typically 15% in most categories, 8% for some), the FBA fulfillment fee (based on size tier and weight — a standard small item runs roughly $3–5), monthly storage fees (which spike ~3x in Q4), plus optional inbound placement fees. A $30 product can easily carry $12–14 in Amazon fees before you've accounted for your product cost, shipping to Amazon, or PPC.
The FBA calculator here models all of it: referral fee by category, fulfillment by size tier, storage, and your landed cost — then shows net profit and margin. The rule experienced sellers use: aim for 30%+ net margin after all fees, because PPC, returns (2–5% is normal), and price wars will eat the rest. If the calculator shows 12% margin at your target price, the product doesn't work. Better to learn that in 30 seconds than after a $5,000 inventory order.
eBay and Etsy: death by a thousand fees
eBay's headline is a ~13.25% final value fee on most categories (plus $0.30 per order), calculated on the total including shipping and tax — not just the item price. Sell with free shipping and you're paying fees on money you never kept. The eBay fee calculator shows the real take-home including payment processing, so "free shipping" becomes an informed choice instead of a hope.
Etsy is sneakier: a $0.20 listing fee, 6.5% transaction fee, payment processing (~3% + $0.25), and for sellers doing over $10k/year, a mandatory 12% offsite-ads fee on sales from Etsy ads. Stack them on a $40 sale and you're near $8 in fees. The Etsy calculator itemizes each one — most sellers are stunned the first time they see the breakdown.
Payments, print-on-demand, and pricing
Taking payments isn't free either. The payment fee comparison pits Stripe, PayPal, and Square against each other on your actual volume — at small ticket sizes, the fixed $0.30 per-transaction component dominates, which is why micropayments need different pricing. For print-on-demand, the calculator works backward from the base cost (shirt + print + shipping) to the retail price you need for your target margin — POD only works if you price with the math, not vibes.
Running discounts? The discount & reverse markup calculator stacks multiple discounts correctly (two 20% discounts isn't 40% off — it's 36%) and reverses margins: if you need 50% margin, what do you charge? Restaurant owners get the food cost calculator — the industry targets 28–35% food cost, and menu pricing without it is guessing.
Creators: what should you charge sponsors?
The sponsorship rate estimator is for YouTubers and TikTokers getting their first brand deals. Rough benchmarks: dedicated YouTube integrations often price at $20–50 per 1,000 views (CPM varies wildly by niche — finance pays multiples of entertainment), while TikTok sponsored posts trend lower per view but move faster. The estimator gives you a defensible number so you stop accepting "$50 and free product" for 100,000 views. Know your worth, then add 20% — brands negotiate down, never up.
The $10,000 mistake: a cautionary tale
A seller I know — let's call him Dan — found a "winning product" on Alibaba: $8 landed cost, selling at $24.99 on Amazon. "That's a 68% margin!" he said. Then reality: 15% referral fee ($3.75), FBA fee for a standard-size item ($4.50), inbound shipping and prep ($1.20), PPC at $1.10 per click converting at 12% ($9.17 in ad spend per sale). Actual profit per unit: under $2. After returns and a price war, he lost money on 1,200 units.
Every one of those numbers was knowable in advance — and that's exactly what the FBA calculator models. The lesson isn't "don't sell on Amazon." It's: the fee stack is knowable, the ad costs are estimable, and a product that needs everything to go right isn't a business, it's a lottery ticket. Run the pessimistic case first. If it survives that, you might have something.