Automated Customer Thank-You Emails

I want to set up a fully automated workflow that sends personalised thank-you emails to my customers and clients straight from Gmail. The goal is simple: each time I complete a sale or deliver a service, the recipient should immediately receive a friendly, on-brand message of gratitude without any manual effort on my part.

Here is what I need:
• A clean, reusable HTML email template that reflects my branding and looks good across major mail clients.
• A Google Apps Script (or another reliable Gmail-compatible solution) that triggers the send, pulls customer details from a Google Sheet or similar source, and inserts dynamic placeholders such as name, product/service purchased, and date.
• A brief walkthrough or set-up guide so I can adjust the copy, add new recipients, and monitor delivery results on my own after hand-over.

Acceptance criteria: the template renders correctly in Gmail web and mobile, at least one test batch reaches my inbox without hitting spam, and the script is clearly commented so I can tweak it later.