What I have seen work most reliably is to log in, add another item to your cart (doesn’t matter what) then go to shopping cart. Once there you can remove the additional unwanted item and the original item you wanted will be there and you can check out.
It does not always work like that. This happened to me with two Extravaganza bundles and Moonshines Diner Bundle previous week: I put all 3 bundles in the cart. Yet when I entered the cart, only one Extravaganza bundle was in there, the other one and Moonshines were not. I tried to add them again, but I got the known error message stating that you cannot add an item to cart twice. I logged out and logged in again. I removed the only Extravaganza visible in the cart, then added another random item in the cart (since I already heard of the solution you are suggesting), but no change, the remaining Extravaganza and Moonshine were still not visible in the cart.
After a minute or two a mirracle occured - I was able to add Moonshine and both Extravaganzas in the cart after all (suddenly the message about item being added twice did not appear although I did not remove said invisible items from cart). Just to make sure that no surplus items are in my cart, I removed all items again from cart, it was empty.
Then I bought some other DAZ originals (non-Extravaganza, non-Moonshines), check-out was without problem. Yet after I checked-out and started downloading the bought stuff, I noticed that the store shows there are 2 items in my cart, although I did not add any. I opened the cart and guess what was in there - the previously invisible Extravaganza and Moonshines… I wonder how they managed to not be in the cart when I paid for the order, yet at the same time be in the cart to show up after check-out? Oh, the wonders of Magento software…