Invoicing App for Data Analytics Consultancies: Milestones, Retainers and PO Numbers

An invoicing app for a data analytics consultancy should bill the way engagements are sold: milestones from a statement of work, monthly retainers with days used against the allowance, and day-rate work, each carrying the PO number corporate clients insist on. Mythex builds that app from a description, with PDF invoices, quotes that convert into invoices and a list of what is overdue.

Who it's for

Small analytics and BI consultancies, and independent data consultants, who are tired of invoices bouncing back from enterprise accounts payable and of working out retainer usage by hand at month end.

What you can build

Client records with legal entity, billing address, PO number and cost centre, pulled into every invoice
Milestone invoices showing the percentage of the SOW due at each stage
Retainer invoices listing days used against the monthly allowance
Pass-through lines for licences or tools bought on the client's behalf
Quotes by phase that turn into milestone invoices once signed
A paid, unpaid and overdue list with the total due this month

What a consultancy invoice must carry

Enterprise accounts payable teams reject invoices for small reasons: the wrong legal entity, a missing PO number, no cost centre, a project name that does not match the purchase order. Each rejection adds another payment cycle. Store those details on the client record once, check them when the PO is issued, and let every invoice fill them in. Add the SOW reference and milestone name, so the approver can match the invoice to the contract without emailing you.

Client legal entity and billing address
PO number and cost centre
SOW reference and milestone name
Payment terms as agreed in the contract

Billing milestones, retainers and day rates in one place

Engagements rarely use one model. A pipeline rebuild might be billed 30 percent on signing, 40 percent at build complete and 30 percent at handover, followed by a monthly support retainer. Set up each engagement with its billing model and the app knows what to invoice next. For retainers, log days as you work and the invoice shows used, remaining and any overage at the agreed rate.

Mistakes to avoid

Do not mix pass-through costs into your fee lines; a licence bought for the client belongs on its own line, often without markup, so finance can code it. Do not invoice a milestone before it is signed off; record the sign-off date and attach it. And do not let a retainer's unused days roll over unless the contract says so, or the allowance quietly grows each month.

Separate pass-through lines
Sign-off recorded before a milestone invoice
Rollover rules written into the retainer

What to enter first

Add your current clients with their billing details and live POs, then each open engagement with its billing model and remaining milestones. Import past invoices from a spreadsheet so numbering continues where it left off. After a month, the outstanding list shows who pays late, which is worth knowing before you agree terms on the next SOW.

A prompt to start from

Build an invoicing app for my data analytics consultancy. Clients have legal entity, billing address, PO number, cost centre and payment terms. Each engagement is billed by milestones against an SOW, a monthly retainer with a day allowance, or day rate. Invoices pull the client details automatically, list pass-through costs separately, export as PDF, and show as paid, unpaid or overdue. Quotes by phase convert into milestone invoices.

Questions

Can it produce PDF invoices?

Yes. Every invoice gets a printable PDF with your logo, the client's PO number and the milestone or retainer details. You can download it or attach it to an email yourself.

Does it handle VAT or sales tax?

You tell Mythex the tax rates you charge and when they apply, for example to clients in different countries, and it adds them to the invoice. Check the rules with your accountant; Mythex does not decide what tax you owe.

Can I track retainer days used?

Yes. Log days against an engagement and the retainer invoice shows days used, remaining and any overage at the rate you set.

Do I need to know how to code?

No. You describe your billing models and Mythex builds the app with a live preview. Adding a field such as cost centre is one chat message.

How much does it cost?

You can build and publish on the Free plan without a card, using daily credits. Pro is $25 a month for code editing, export, GitHub, a custom domain and more credits.

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