How do I keep post-purchase upsells from cannibalizing initial conversion?

I'm struggling with implementing post-purchase upsells in my e-commerce store without negatively impacting my primary conversion rates. Every time I try to add an additional offer after a customer completes their initial purchase, I'm worried it might seem pushy or reduce the likelihood of customers completing their first transaction. I need strategic guidance on how to design upsell funnels that enhance revenue without undermining the primary purchase experience or making customers feel overwhelmed. What are the best practices for creating post-purchase offers that complement rather than compete with my main conversion goals?