Selling a single book through a single platform is simple to track. Selling eighteen books, direct through Gumroad and wide through Draft2Digital into dozens of retailers, in at least two currencies, on payout schedules that don't line up with each other, is not — and the gap between those two situations catches almost every self-publisher who scales past a first release.
The system that's kept this manageable for me is unglamorous on purpose: a single running spreadsheet, updated monthly rather than reactively, with one row per platform per month rather than trying to reconcile every individual sale. Gumroad's dashboard exports cleanly. Draft2Digital's reporting rolls up sales across every retailer in its network into one view, which is one of the real practical advantages of wide distribution over managing five separate retailer dashboards individually. Converting everything to a single currency at the point of entry, rather than trying to do it later, saves a genuinely significant amount of monthly admin.
The habit that actually matters is the monthly cadence itself, not the spreadsheet's design. Twenty minutes on a fixed day each month, done consistently, is worth more than an elaborate tracking system used sporadically. The unglamorous, repetitive discipline of proper record-keeping is something 29 years in the police left me with no illusions about skipping — it's tedious precisely because it works, the same way it works for evidence logs as it does for royalty statements.
For UK-based authors specifically: every one of these income streams counts toward self-assessment income, regardless of which currency it arrived in or which platform paid it out, and HMRC doesn't particularly care that Gumroad and Draft2Digital use completely different reporting formats — the record-keeping obligation is yours to reconcile, not theirs to simplify for you. Keeping the monthly log current makes the annual tax return a transcription exercise instead of a January archaeology project through a year of scattered platform emails.
None of this needs to be sophisticated. It needs to be consistent, and it needs to exist before you need it — which, for most authors, is earlier than it feels like it should be. The month you tell yourself you'll start tracking properly "once sales pick up" is usually the month it stops being optional.