Embedded checkout applies coupon amount from a previously viewed plan (shows €0.00 total)

Hello,

We use Pabbly Subscription Billing with the embedded checkout on our website, and we have found an intermittent problem with coupon calculation.

Setup

• Checkout page: 67fc21d8bb903ffc725606e5 (product “BrickControl (eur-es)”)
• Embedded with: embed.js?_m=67fc21d8bb903ffc725606e5&preferred_plan=<plan_id>&coupon_code=PROMO3M
• Each plan has its own page on our site, but they all share this same checkout page:
  • Basic monthly (€98/month): preferred_plan=67fa354fbb903ffc725598c6 → https://www.brickcontrol.com/es/precios/basic-mensual/
  • Business monthly (€198/month): preferred_plan=67fa36a9bb903ffc72559960 → https://www.brickcontrol.com/es/precios/business-mensual/
• Coupon PROMO3M: 50% off for 3 months.

Steps to reproduce

1. Open the Business monthly page. The checkout correctly shows €198.00 with a coupon discount of -€99.
2. In the same browser session, open the Basic monthly page.

Expected: Subtotal €98.00, discount -€49, VAT €10.29, total €59.29.

Actual (intermittent): Subtotal €98.00, discount -€99, VAT -€0.21, total €0.00.

The discount amount is carried over from the previously viewed plan (50% of €198), even though the iframe URL requests the Basic plan. Reloading the page usually corrects it. We have seen it both in our own tests and in a real browser session.

Questions

1. Can you fix the coupon calculation so it always uses the plan currently selected in the checkout?