The submission portal is where papers go to be rejected by a form. Missing anonymization. Wrong file format. Supplements attached to the wrong slot. None of it has anything to do with your research. All of it can kill a submission.
This is the one category where I'll happily recommend the free stuff. A paper submission checklist app, the free kind, covers the same ground the paid ones do, because the ground is a checklist. It's not a moat. Anonymized PDF? Correct template? Page limit respected? References formatted? Figures embedded and legible? Author list complete? These are yes-or-no questions, and yes-or-no questions don't need a subscription.
Compare free options on three things. Venue coverage: some checklists are generic, some know specific venues' quirks. Generic is fine for journals. Specific helps for conferences with weird rules. Export: can you print or share the checklist for the coauthor who will definitely skip steps? And reminders, because a checklist you forget to open is a very organized paperweight.
The one paid upgrade worth considering is a paper formatting checker SaaS, which is a different animal entirely. A checklist asks you if you did things. A checker verifies you did them. Before a flagship submission, run both. The free app for the process, the checker for the proof.
An academic paper formatter app handles the writing stage. The checklist handles the upload stage. Between them, conference submission deadlines stop being scary for a very fixable reason. Most submission failures are logistics, not science.
And for the final human pass, confjournal.fyi runs paper formatting and submission checks so the portal has nothing to complain about.