TheCheckout module that displays the checkout tunnel on a single page can cause incompatibilities with the Floa module and, in general, with all payment modules.
In most cases, these modules redefine part of the purchase tunnel, making it impossible to update payment amounts following carrier selection.
Alter the following files with the changes made (look at files in the zip) :
/modules/thecheckout/views/js/front.js - Lignes 767 à 798
/modules/thecheckout/views/js/front.js - Lignes 842 à 861
/modules/thecheckout/views/js/payment.js - Lignes 261 à 279
The changes are contained between these comments and must be placed in the same place in the code :
// ** UPDATE 202 FLOAPAY */
// ** END UPDATE 202 FLOAPAY */
Depending the installed version, the lines present in the attached files will not correspond, you will need to adapt them or contact the module publisher if you have any doubts.
Second method : to avoid
Replace files of your Checkout module with those included in the zip :
/modules/thecheckout/views/js/front.js
/modules/thecheckout/views/js/payment.js
To avoid because if you have a different version of the checkout module for which the fix has been developed (v3.3.1), you can loose features or fixes made with the update.
WARNING : Applying this fix implies you do not update the checkout module or you must report each time the fix.
The best way is to contact The Checkout team in order to ask him to include the fix in his next release.