Since I don't require long-term contracts, I send and invoice and expect to get paid in the beginning of each month.
This way you as a client are free to continue or discontinue our relationship at the end of each month. And I have to earn your business each month.
I also manage Bing Ads (now called Microsoft Ads).
The Bing Ads monthly fee is 20% of the current Google Ads monthly management fee.
There's not initial tune-up fee portion for the Bing Ads.
It's simpler both for myself and the client to know what exactly the monthly fee be at any given time.
If I was charging a flat percentage of ad spend, some clients may feel that a suggestion to increase budget in a particular campaign is being suggested with the ulterior motive of being able to bill the client more at the end of the month.
With brackets, there is always room to spend more without any increase in fees.
It's simplifies the billing process to charge for a combined spend across all active campaigns instead of charging for each campaign type (Shopping, Search, Remarketing, Display) separately.
Additionally, since I manage PPC accounts with a high level strategy, monthly budget it very often shifted to different campaigns based on where performance gains will be most likely realized.
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Your information is safe I won't ever spam of sell your contact details. I hate BS like that as much as you do.