A subscription bought in the Sendinvo app covers one paid person, and Apple cannot change that number. When you take someone on, you move the billing to the web, where a subscription covers you plus as many paid people as you need. You keep the period you have already paid Apple for, and you are never charged for both at once.
Sendinvo is £20 a month, and each paid person after you is £5 a month. Viewers are free.
It is the whole of Sendinvo for one paid person: invoices, quotes, credit notes, reminders, recurring invoices, card and bank payments. What it cannot do is grow.
An App Store subscription is sold as a single item at a single price. There is no quantity to change, so there is no way to add a second paid person to it. If you invite someone as an Admin or a Member while the App Store is billing you, Sendinvo will tell you the same thing: paid team members are not available on an App Store subscription, and you can still invite Viewers, who are always free.
Web billing is the same subscription with a number on it. You, plus each Admin or Member you add, each at £5 a month.
Sign in at sendinvo.com, open Profile, and choose Move to web billing in the Billing section. It is on the web only, because the app cannot start it.
You go through the usual checkout and enter a card. Nothing is charged to it yet: the first payment is set for the day your App Store period ends.
This part is yours to do, and it is the one that matters. Apple goes on renewing until you say otherwise, and Sendinvo cannot turn it off for you.
Paid invites open as soon as the move is set up, not when it finishes. You do not have to wait out the App Store period to get someone working.
You have paid Apple up to a date. Sendinvo keeps billing you through Apple until that date, and web billing takes over the day after. Moving early does not shorten it and does not refund it.
The card you add at checkout is held for the changeover. Its first charge is the day your App Store period ends, so the two never overlap.
This is the one way to end up paying twice, and it is not something Sendinvo can prevent. Apple renews a subscription until the person who bought it turns renewal off, and only they can do it. Turn it off and you keep the rest of the period you have paid for.
Open Settings, tap your name at the top, then Subscriptions. Choose Sendinvo and tap Cancel Subscription. That stops the next renewal; it does not end the period you are in, and it does not touch your Sendinvo account or your invoices.
You can get there from Sendinvo too: the Billing section on the web links straight to your App Store subscriptions.
If you would rather not move yet, do nothing. An App Store subscription carries on as it is, and you can add Viewers for free in the meantime.
Only the person who bought it can, and Sendinvo has no way to. If you are closing your account, turn off renewal on your iPhone as well.
Billing is the only thing changing. Your invoices, clients, quotes and settings are the same, on the same account, whichever way you are paying and whichever device you are on.