BFCM is around the corner, it is the most powerful seasons to boost sales and skyrocket your revenue. I've recently published an app called DoubleC Upsell & Cross sell (which is totally free) on the app store which can help sellers to offer flash deals of special products: apps.shopify.com/flash-deal
Basically, our app will show a popup with discounted products or variants on the cart page, product page and thank you page. Up to 3 products can be shown at the same time so you can offer relevant or frequently-bought-together products. There are multiple offer layout available and you can also offer the deal on multiple pages, which can boost your average order value.
The popup has a countdown timer which create a sense of urgency. You can also fully customize the layout and text.
Also, we are working on the product bundles and volume discount function. I want to know if this is what you want from an upsell app, and if not, what more do you expect? Your suggestion will mean a lot to us.