Best way to handle payments and payouts for small “online used clothing” shops?

I’m running a small ecommerce project focused on online used clothing and I’m looking for a simple, reliable way to handle payments and payouts as the shop grows. Right now I’m mostly concerned with keeping things secure, reducing failed payments and making it easy to track orders and refunds for customers in both the UK and USA.

For those of you who have integrated payment APIs into small stores, which solutions or patterns have worked best for you. And are there any common mistakes to avoid when setting up payments for a niche shop like online used clothing?