Whilst smart bidding and automated bidding strategies certainly have their place, they can be incredibly unpredictable, especially in the beginning. It is recommended for an automated bidding strategy that there is plenty of data on the campaign. A new Google Shopping campaign will have no performance data, therefore, when using a Target CPA, Max Conversions, Target CPA, etc, bidding strategy Google may bid extremely high and exhaust your budget early in the day with no results.
As such, I would always recommend starting on a manual bidding strategy and moving this over to an automated bidding strategy (if you wish) once you are receiving more than 50 conversions per month and there is at least 60 days of performance data on the campaign. N.B, when you change bidding strategy to automated from manual, Google will enter a “learning period” where results will again be unpredictable during this phase. The more conversion data your account has, the quicker the learning period will be completed.