For a small team already on a Microsoft 365 Business plan, the $18 per user per month promotional rate for the Copilot Business add-on locks in only through 30 Jun 2026 before rising to the $21 standard rate, so the cost-rational move is to decide before the deadline, but only on the seats whose actual Word, Excel, Outlook and Teams usage clears the per-seat math, because the discount applies only to the first year and only to seats licensed before the deadline.

Anchored on Microsoft's 2 Dec 2025 Microsoft 365 blog (Copilot Business GA, $21/user/month annual commitment, up to 300 seats, Jared Spataro CMO-AI-at-Work quote naming Word/Excel/PowerPoint/Outlook/Teams) and Microsoft's Tech Community deadline notice (existing M365 Business customers hold a promotional $18/user/month through 30 Jun 2026, first year). VERIFIED 2026-06-05 via microsoft.com/en-us/microsoft-365/blog (2 Dec 2025 GA post, $21 standard, 300-seat cap, Spataro quote) and techcommunity.microsoft.com (30 Jun 2026 deadline notice). Precision caveat carried in the body: the $18 standalone add-on offer is distinct from an earlier, now-expired Dec 2025 to 31 Mar 2026 bundle promotion (do not conflate). The $36/year/seat figure is arithmetic on the cited $3/user/month difference, not a Microsoft statistic. Operator-register advisory; the claim is decide-before-the-deadline-but-on-usage, NOT a blanket endorsement of Copilot. 30-day cadence, set to land just after the deadline. Triggers: (1) Microsoft extends the $18 rate past 30 Jun 2026; (2) Microsoft changes the $21 standard price or the 300-seat cap; (3) a packaging change folds Copilot into the base Business plans (as E7 did for enterprise tiers). Siblings: OPS-003 (Claude Pro vs ChatGPT Plus for a solo founder); the solopreneur stack-consolidation read; AM-202 (Microsoft 365 E7 enterprise tier read).