First-time buyer coupon on web shop: incremental margin worksheet (labelled synthetic)
Synthetic worksheet for FTB web-shop coupons after payment fees and store-cannibalisation. Official Play service-fee page cited for store counterfactual; no invented averages.
First-time buyer (FTB) coupons look free until payment fees and cannibalised store IAP are counted. This worksheet uses illustrative inputs. The store counterfactual fee is a reader input constrained by Google Play service fees—not a frozen “30% forever” slogan.
Equations
web_net = (shelf_price - coupon) * (1 - vat_fraction) * (1 - psp_fee_fraction)store_net_counterfactual = store_list_price * (1 - store_fee_fraction)incremental_vs_store = web_net - store_net_counterfactualcannibalised = 1if the buyer would have purchased the same SKU via store IAP in the window (reader flag)
If cannibalised = 1, contribution versus doing nothing on web is web_net - store_net_counterfactual (often negative when coupons are deep). If cannibalised = 0, contribution versus no purchase is web_net minus allocated UA cost (separate input).
Synthetic example
| Input | Value |
|---|---|
| Shelf price | 19.99 |
| Coupon | 5.00 |
| VAT fraction | 0.20 |
| PSP fee fraction | 0.029 |
| Store list | 19.99 |
| Store fee fraction | 0.15 |
| Cannibalised flag | 1 |
web_net = (19.99−5) × 0.80 × 0.971 = 11.6442
store_net = 19.99 × 0.85 = 16.9915
incremental_vs_store = -5.3473
| Output | Result |
|---|---|
| web_net | 11.6442 |
| store_net_counterfactual | 16.9915 |
| incremental_vs_store | -5.3473 |
Facts versus limits
| Fact | Limit |
|---|---|
| Play documents programme service fees | Exact fee is programme- and date-specific |
| Synthetic coupon math is reproducible | Not proof your FTB coupon destroys margin |
| Vendor web-shop blogs were candidates | Official fee definition preferred over marketing posts |
Download the CSV.
