The moment someone's decided to buy, they're unusually open to buying a little more — if the little more genuinely fits. A relevant add-on at checkout raises what each customer is worth, and you've already done the hard part of convincing them to pay at all.
Offer the obvious companion
The best upsell is the thing that naturally goes with what they're already buying — the templates that pair with the guide, the extra pack, the faster support. It should read as 'oh, useful', not 'they're squeezing me'. Relevance is the whole difference.
Keep it optional and light
One clear, easy-to-decline add-on — not three, not a guilt trip. A gentle 'want to add this?' lifts orders without souring the sale. Push too hard and you risk the whole purchase you'd already won.