Help designing an e-commerce ‘build your own product’ interface

I'm designing a 'build your own product' interface which allows a user to upgrade a base product. Currently I'm unable to get feedback from our users, but I did share the design internally.

For security purposes I've had to replace the actual content with filler text.

One of pieces of feedback I received was that the base product column and performance upgrade column felt disconnected. I tried adding a plus sign icon between them, but it looked silly.

Any suggestions about how to remedy this issue or suggestions how to improve this design would be greatly appreciated.