The move to electronic invoicing is being phased in, and most of the attention is going to choosing a provider. That is the smallest part of it.
Your data has to be right first
Electronic invoicing transmits what your system holds. If customer tax identifiers are incomplete, if the same customer exists three times under different spellings, if product codes and VAT treatments were set up years ago and never reviewed, those errors stop being internal.
Clean the master data before go-live. It is unglamorous, it takes longer than the software configuration, and it determines whether the first month is quiet or not.
Map every way you issue an invoice
Most companies have more than one. The main system, a spreadsheet used by one department, manual invoices for a particular customer, credit notes handled informally, and whatever happens when someone is on holiday.
Each of those paths has to end up in the same place. The ones nobody mentions in the project meeting are the ones that produce gaps.
Decide how corrections will work
Mistakes will happen. Decide in advance how a wrong invoice gets corrected, who is allowed to do it, and how the link between original and correction is preserved. A correction process invented under pressure is how a small error becomes an unexplainable one.
Reconcile from the first month
Compare what your sales ledger says you issued against what was actually transmitted, every month, and resolve differences before the period closes. Configuration errors are common at the start and cheap to fix immediately.
Plan for the people, not just the system
Whoever raises invoices needs to understand what changed and what to do when something is rejected. Training that happens the week before go-live is training that happens during go-live.
Treat it as a process change with a software component, and the deadline is manageable. Treat it as a purchase, and it will not be.