Fix WooCommerce Checkout Bug + Add Stripe Payment Gateway — Urgent

We run a WooCommerce store selling custom pet supplies and we're losing
sales because our checkout is broken. Two major issues:

1. CHECKOUT BUG: After the last WooCommerce update (v8.6), the checkout
page throws a fatal error when customers select "Ship to a different
address." About 30% of our orders use this option so it's killing our
revenue. The error log shows a PHP deprecated function warning related
to our shipping plugin (WooCommerce Advanced Shipping).

2. STRIPE INTEGRATION: We currently only accept PayPal. We need Stripe
added as a payment option (credit/debit cards). We already have a Stripe
account with API keys ready. Need it set up with:
- Standard card payments
- Apple Pay + Google Pay enabled
- Stripe's built-in fraud protection (Radar)
- Test mode verified before going live

Additional cleanup while you're in there:
- The "Apply Coupon" button on the cart page doesn't work (returns error
"coupon not found" even for valid codes)
- Order confirmation emails are going to spam — probably a header issue

Tech details:
- WordPress 6.4.3
- WooCommerce 8.6.1
- PHP 8.1 on SiteGround
- Theme: Flavflavor (child theme active)
- ~450 products, ~80 orders/week

We'll give you WP admin + SiteGround access + Stripe API keys immediately
after awarding. Please work on a staging copy first and push to live only
after we verify