Numbers MAX Formula unexpected result

Although one cannot style individual characters in a formula, one feature of Apple Numbers is the canvas. One can take advantage of this by adding tables and text objects anywhere.

The above screen shot uses a static (as the OP's "Invoice terms" text is hard coded in their formula example, it is assumed static) text field placed next to the table (a single cell table with no border, in this case) containing the cell with the formula.

The OP's use of the CONCATENATE function is unneeded as the ampersands do the same job. To use CONCATENATE in this case:

=CONCATENATE(Terms,", please pay by...")

Where Terms is the reference to the cell containing the Terms text.

Detailed screenshot:

By using a number for the days, this can be used in a calculation against the Invoice date to determine the date payment is due. But that is a topic for another question. ;)

Note: This works with all versions of Numbers.

To create a running total, if that is what you are asking, use two columns. One for numbers to subtract, the other for the running total. Add another column for numbers to add if desired.

In the example below, the starting number is in cell reference B2 and the number to subtract from it is in cell reference A3. Cell B3 enter this formula: =B2 - A3 then, click and drag the yellow dot (shown in the illustration) to autofill the formula down into the cells below. Numbers will update the cell references automatically.

Of course, this can be improved upon but this is the basic formula to answer the OP's question.