The embed is on in the theme you publish
A deal app built as a theme app extension shows nothing until its app embed is on, and the embed is switched per theme. Open the theme editor on the theme customers will see, go to App embeds in Theme settings, and check the toggle there. If you are about to publish a different theme, check that one too, because each theme keeps its own setting.
In TreStack Bundles the embed is called Bundle deals, and Home says whether the store widget is enabled or disabled, with an Activate app embed link that opens the toggle. The optional Bundle deal app block is only needed if you want the widget somewhere other than above Add to cart.
Then open the product page itself. The widget should sit above the Add to cart button, and changing the variant should update it. On a theme the app does not know by name, placement still works but the theme's own variant picker, quantity box and price are not kept in step, which is better seen by you than by a customer.
Every market and currency you sell in
A deal priced in your store currency is converted for shoppers who pay in another one. Check the result rather than assuming it: open the product page from each market you sell in and read the tier prices the way a shopper there would. A rounding rule, such as ending every price in .99, is applied in each shopper's currency, so read the rounded figure in each one.
In TreStack Bundles amounts are entered in the store currency and converted with Shopify's rate. A deal limited to one market can be priced in that market's own currency, which is exact there. If no usable rate exists, the shopper gets no discount rather than a wrong one, so an offer that quietly disappears in one market is the thing to look for. Country previewing in the editor shows the widget in another country's currency.
Language is the other half of a market. The widget's built in texts come in English and 19 other languages, machine translated. Your own texts, including the discount name shown at checkout, are translated per language on the Translations page. Read the checkout line in each language you sell in.
Every other discount on the store
List what is already live: automatic discounts, discount codes in circulation, shipping offers, and anything another app created. Shopify allows 25 active automatic discounts per store, counted across all apps, and each published deal in TreStack Bundles uses one of them.
Then decide how the new deal meets them. TreStack Bundles sets every deal's discount to combine with product, order and shipping discounts, and there is no setting to change that, so make sure any code you are running is one you are content to see on top of a tier price. If the store runs a second bundle app, keep it off the products this deal targets: two bundle apps on one product both change the quantity, and checkout applies one product discount per line.
One product, one deal. The widget shows a single deal per product, but every published deal's discount runs at checkout, so a product in two deals can be priced by the one the shopper never saw. The editor warns when a product is already in another deal. Treat that warning as a stop sign.
One real purchase, on a phone
Most shoppers will meet the deal on a phone, so do the last check there, on mobile data rather than office wifi. Pick the middle tier, add it, open the cart, and go as far as the payment step. Check that the cart shows the right lines, that the discount name reads the way you wrote it, and that the total matches your own arithmetic.
Then try to break it. Change the quantity in the cart and confirm the price follows: in TreStack Bundles three reduced to two gets the two item price, and below the lowest tier there is no discount. Buy it now is hidden whenever the chosen offer is not one plain line of the page's variant, such as a bundle or a tier with a gift, so those shoppers go through the cart. Check that the cart page or drawer is ready for them.
After publishing, allow about a minute for the deal to reach the storefront. View in store opens the live product page, so it shows what is published, not unsaved edits. Adding ?bndl=off to the product page address hides the widget, which gives you a quick before and after if anything on the page looks wrong. TreStack Bundles is coming to the Shopify App Store, and the docs below follow the same order as this list.