How do I sell more without feeling pushy?

Feeling pushy when you try to sell more, and so not doing it, is a sign of decency, not a flaw. You do not want to pressure people, and that instinct is a good one. But refusing to offer anything can quietly cost you, and it assumes the customer would not have wanted more. Offering is not pushing. Pointing out something that fits what they are already buying is helpful. The pressure you fear comes from not taking a no, not from the first friendly suggestion. Make the offer once, mean it, and let them decide without any weight behind it. Next customer, mention one thing that honestly goes with their purchase, then leave it with them. A single genuine suggestion serves them and steadies you.