Fix issue with syncing shipping rates with more than two decimals

[jorgemd24]

Changes proposed in this Pull Request:

While working on #2527, @mikkamp found an issue where shipping rates with more than two decimal places were not syncing correctly. This PR makes sure that rates are always synced with exactly two decimal places.

Screenshots:

Detailed test instructions:

1. Go to WC -> Settings -> Shipping
2. Go to the flat rate section, right-click on "Edit," and open it in a new tab to access the classic Shipping settings screen.

3. Set a price with more than two decimal places.

4. Go to GFW -> Dashboard and click "Edit" in the free listings section.

5. Click "Save Changes" and verify that there are no errors and that the shipping settings are correctly updated in your Merchant Center.

Additional details:

Changelog entry

Fix - Issue with syncing shipping rates with more than two decimals

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.0%. Comparing base (93b4180) to head (708910d).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##             develop   #2573     +/-   ##
===========================================
+ Coverage       64.5%   65.0%   +0.5%     
  Complexity      4591    4591             
===========================================
  Files            796     475    -321     
  Lines          22992   17907   -5085     
  Branches        1234       0   -1234     
===========================================
- Hits           14837   11646   -3191     
+ Misses          7982    6261   -1721     
+ Partials         173       0    -173     

Flag	Coverage Δ
js-unit-tests	?
php-unit-tests	65.0% <100.0%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/Shipping/ShippingRate.php	88.0% <100.0%> (ø)

... and 321 files with indirect coverage changes

[martynmjones]

Hey @jorgemd24, thanks for the fix.

Confirmed that no errors are triggered when saving the campaign settings and the value is correctly synced with Google.