This is an issue found with some themes.
We have built SellUp to Upsell and communicate clearly that the item has been added to the cart within the remits of what we, the app provider, can do. This is done by changing the text on the button after a click. In some cases, the theme is using a static cart. The contents will not be shown until the page is refreshed or the customer goes to their cart.
This is simply down to your theme. It may utilise a mini-cart, a side cart, a floating cart and so on. SellUp adds the product via the Cart API, an industry standard practice.
If your store redirects to the cart after a product is added then you will not see this issue
If your product page refreshes after an add to cart by default, you will not see this issue.
If you are affected by this and wish to get your theme updating to relay this issue, please read on.
For the most part, this has little to no effect on the customer as they have been communicated that the item is in their cart and will eventually (hopefully) get to the checkout.
Some themes can be modified to listen to cart changes and you can also use some JS to force a refresh/update of the AJAX cart or current page after an item has been added. You will need to request your developer alters this for you, seek help from the theme developer or get a Shopify Expert to assist if you need your mini cart to refresh. Our policy on why we can’t do that is here.