Start with contribution before advertising
The maximum advertising spend an order can absorb is the amount left after its other included costs. This is a break-even limit, not a recommended budget. Spending the entire amount leaves nothing for omitted overhead or profit. Begin with the same revenue definition in every calculation, especially when reports treat shipping and tax differently.
Assume a $60 order, $22 product cost, $6 fulfillment and an illustrative payment-and-platform fee of 10% plus $0.50. Fees are $6.50. Before advertising, the order contributes $25.50. Spending $25.50 to obtain it would leave zero under these assumptions.
Translate between acquisition cost and ROAS
Customer acquisition cost per order is advertising spend divided by attributed orders. Return on ad spend, or ROAS, is attributed revenue divided by advertising spend. They describe different sides of the same campaign and are only comparable when the reporting periods and attribution rules match.
For the $60 order, break-even ROAS is $60 ÷ $25.50, approximately 2.35. At a ROAS of 3, advertising costs $20 per order and modeled profit is only $5.50. A campaign producing three dollars of revenue for each advertising dollar can therefore leave a narrow contribution rather than a large profit.
Add a profit target
Suppose you want $12 per order after advertising. Subtract that from the $25.50 pre-ad contribution: your maximum acquisition cost becomes $13.50. Required ROAS becomes $60 ÷ $13.50, approximately 4.44. This is a planning threshold for this specific basket and cost structure.
If repeat purchases are part of your strategy, model them separately using observed repeat-order contribution and timing. Do not justify a loss on the first order using an assumed lifetime value that has not been demonstrated. Cash spent today and revenue that might arrive later are not interchangeable.
Use actual campaign data carefully
In the e-commerce calculator, choose fixed-dollar advertising to test a known acquisition cost. Percentage mode is useful when advertising is deliberately modeled as a share of receipts. Do not enter both a campaign allocation and the same advertising expense inside overhead, or you will count it twice.
Review multiple orders, not just one favorable basket. Discounts, returns and different product mixes change contribution. Also check whether attributed sales include customers who would have purchased without the campaign. FeeMargin can calculate a cost scenario; it cannot establish that an advertisement caused a sale or reconcile competing attribution systems.